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    <H1>Developer's Frequently Asked Questions (FAQ) for
    PostgreSQL</H1>

    <P>Last updated: Sat May  5 10:30:33 EDT 2007</P>

    <P>Current maintainer: Bruce Momjian (<A href=
    "mailto:bruce@momjian.us">bruce@momjian.us</A>)<BR>
    </P>

    <P>The most recent version of this document can be viewed at <A
    href=
    "http://www.postgresql.org/docs/faqs.FAQ_DEV.html">http://www.postgresql.org/docs/faqs.FAQ_DEV.html</A>.</P>
    <HR>
    <BR>


      <H2>General Questions</H2>
    <A href="#item1.1">1.1</A>) How do I get involved in PostgreSQL
    development?<BR>
     <A href="#item1.2">1.2</A>) What development environment is required
    to develop code?<BR>
     <A href="#item1.3">1.3</A>) What areas need work?<BR>
     <A href="#item1.4">1.4</A>) What do I do after choosing an item to
    work on?<BR>
     <A href="#item1.5">1.5</A>) I have developed a patch, what next?<BR>
     <A href="#item1.7">1.6</A>) How is a patch reviewed?<BR>
     <A href="#item1.7">1.7</A>) Where can I learn more about the code?<BR>
     <A href="#item1.8">1.8</A>) How do I download/update the current
    source tree?<BR>
     <A href="#item1.9">1.9</A>) How do I test my changes?<BR>
     <A href="#item1.10">1.10</A>) What tools are available for
    developers?<BR>
     <A href="#item1.11">1.11</A>) What books are good for developers?<BR>
     <A href="#item1.12">1.12</A>) What is configure all about?<BR>
     <A href="#item1.13">1.13</A>) How do I add a new port?<BR>
     <A href="#item1.14">1.14</A>) Why don't you use threads, raw
    devices, async-I/O, &lt;insert your favorite wizz-bang feature
    here&gt;?<BR>
     <A href="#item1.15">1.15</A>) How are RPM's packaged?<BR>
     <A href="#item1.16">1.16</A>) How are CVS branches handled?<BR>
     <A href="#item1.17">1.17</A>) Where can I get a copy of the SQL
    standards?<BR>
     <A href="#item1.18">1.18</A>) Where can I get technical
    assistance?<BR>
     <A href="#item1.19">1.19</A>) How do I get involved in PostgreSQL web
    site development?<BR>
     <A href="#item1.20">1.20</A>) Why haven't you replaced CVS with SVN, Git,
    Monotone, VSS, &lt;insert your favorite SCM system here&gt;?


      <H2>Technical Questions</H2>
    <A href="#item2.1">2.1</A>) How do I efficiently access information in
    tables from the backend code?<BR>
     <A href="#item2.2">2.2</A>) Why are table, column, type, function,
    view names sometimes referenced as <I>Name</I> or <I>NameData,</I>
    and sometimes as <I>char *?</I><BR>
     <A href="#item2.3">2.3</A>) Why do we use <I>Node</I> and <I>List</I>
    to make data structures?<BR>
     <A href="#item2.4">2.4</A>) I just added a field to a structure. What
    else should I do?<BR>
     <A href="#item2.5">2.5</A>) Why do we use <I>palloc</I>() and
    <I>pfree</I>() to allocate memory?<BR>
     <A href="#item2.6">2.6</A>) What is ereport()?<BR>
     <A href="#item2.7">2.7</A>) What is CommandCounterIncrement()?<BR>
     <A href="#item2.8">2.8</A>) What debugging features are available?<BR>

     <BR>

    <HR>

    <H2>General Questions</H2>

    <H3 id="item1.1">1.1) How do I get involved in PostgreSQL
    development?</H3>

    <P>Download the code and have a look around. See <A href=
    "#item1.8">1.8</A>.</P>

    <P>Subscribe to and read the <A href=
    "http://archives.postgresql.org/pgsql-hackers">pgsql-hackers</A>
    mailing list (often termed 'hackers'). This is where the major
    contributors and core members of the project discuss
    development.</P>

    <H3 id="item1.2">1.2) What development environment is required
    to develop code?</H3>

    <P>PostgreSQL is developed mostly in the C programming language. It
    also makes use of Yacc and Lex.</P>

    <P>The source code is targeted at most of the popular Unix
    platforms and the Windows environment (XP, Windows 2000, and
    up).</P>

    <P>Most developers make use of the open source development tool
    chain. If you have contributed to open source software before, you
    will probably be familiar with these tools. They include: GCC (<A
    href="http://gcc.gnu.org">http://gcc.gnu.org</A>, GDB (<A href=
    "http://www.gnu.org/software/gdb/gdb.html">www.gnu.org/software/gdb/gdb.html</A>),
    autoconf (<A href=
    "http://www.gnu.org/software/autoconf/">www.gnu.org/software/autoconf/</A>)
    AND GNU make (<A href=
    "http://www.gnu.org/software/make/make.html">www.gnu.org/software/make/make.html</A>.</P>

    <P>Developers using this tool chain on Windows make use of MingW
    (see <A href=
    "http://www.mingw.org/">http://www.mingw.org/</A>).</P>

    <P>Some developers use compilers from other software vendors with
    mixed results.</P>

    <P>Developers who regularly rebuild the source often pass the
    --enable-depend flag to <I>configure</I>. The result is that when you
    make a modification to a C header file, all files depend upon that
    file are also rebuilt.</P>

    <P><I>src/Makefile.custom</I> can be used to set environment variables,
    like <I>CUSTOM_COPT</I>, that are used for every compile.

    <H3 id="item1.3">1.3) What areas need work?</H3>
    Outstanding features are detailed in the TODO list. This is located
    in <I>doc/TODO</I> in the source distribution or at <A href=
    "http://www.postgresql.org/docs/faqs.TODO.html">
    http://www.postgresql.org/docs/faqs.TODO.html</A>.


    <P>You can learn more about these features by consulting the
    archives, the SQL standards and the recommend texts (see <A href=
    "#item1.11">1.11</A>).</P>

    <H3 id="item1.4">1.4) What do I do after choosing an item to
    work on?</H3>

    <P>Send an email to pgsql-hackers with a proposal for what you want
    to do (assuming your contribution is not trivial). Working in
    isolation is not advisable because others might be working on the same
    TODO item, or you might have misunderstood the TODO item. In the
    email, discuss both the internal implementation method you plan to
    use, and any user-visible changes (new syntax, etc). For complex
    patches, it is important to get community feeback on your proposal
    before starting work. Failure to do so might mean your patch is
    rejected.  If your work is being sponsored by a company, read this
    <a href="http://momjian.us/main/writings/pgsql/company_contributions/">
    article</a> for tips on being more effective.</P>

    <P>A web site is maintained for patches awaiting review,
    <a href="http://momjian.postgresql.org/cgi-bin/pgpatches">
    http://momjian.postgresql.org/cgi-bin/pgpatches</a>, and
    those that are being kept for the next release,
    <a href="http://momjian.postgresql.org/cgi-bin/pgpatches_hold">
    http://momjian.postgresql.org/cgi-bin/pgpatches_hold</a>.</P>

    <H3 id="item1.5">1.5) I have developed a patch, what next?</H3>

    <P>You will need to submit the patch to pgsql-patches@postgresql.org. It
    will be reviewed by other contributors to the project and will be
    either accepted or sent back for further work. To help ensure your patch
    is reviewed and committed in a timely fashion, please try to make sure your
    submission conforms to the following guidelines:

    <ol>
    <li>Ensure that your patch is generated against the most recent version
    of the code, which for developers is CVS HEAD. For more on branches in
    PostgreSQL, see <a href="#item1.16">1.16</a>.</li>

    <li>Try to make your patch as readable as possible by following the
    project's code-layout conventions.  This makes it easier for the
    reviewer, and there's no point in trying to layout things
    differently than pgindent.  Also avoid unnecessary whitespace
    changes because they just distract the reviewer, and formatting
    changes will be removed by the next run of pgindent.</li>

    <li>The patch should be generated in contextual diff format (<i>diff
    -c</i> and should be applicable from the root directory. If you are
    unfamiliar with this, you might find the script
    <I>src/tools/make_diff/difforig</I> useful. (Unified diffs are only
    preferable if the file changes are single-line changes and do not
    rely on surrounding lines.)</li>

    <li>PostgreSQL is licensed under a BSD license.  By posting a patch
    to the public PostgreSQL mailling lists, you are giving the PostgreSQL
    Global Development Group the non-revokable right to distribute your
    patch under the BSD license.</li>

    <li>Confirm that your changes can pass the regression tests. If your
    changes are port specific, please list the ports you have tested it
    on.</li>

    <li>If you are adding a new feature, confirm that it has been tested
    thoroughly.  Try to test the feature in all conceivable
    scenarios.</li>

    <li>New feature patches should also be accompanied by documentation
    patches.  If you need help checking the SQL standard, see <a href=
    "#item1.17">1.17</a>.</li>

    <li>Provide an implementation overview, preferably in code comments.
    Following the surrounding code commenting style is usually a good
    approach.</li>

    <li>If it is a performance patch, please provide confirming test
    results to show the benefit of your patch. It is OK to post patches
    without this information, though the patch will not be applied until
    somebody has tested the patch and found a significant performance
    improvement.</li>
    </ol>

    <p>Even if you pass all of the above, the patch might still be
    rejected for other reasons. Please be prepared to listen to comments
    and make modifications.</p>

    <p>You will be notified via email when the patch is applied, and
    your name will appear in the next version of the release notes.</p>

    <H3 id="item1.6">1.6) How is a patch reviewed?</H3>

    <p>Patch committers check several things before applying a patch:</p>

    <ul>
    <li>Patch follows the SQL standard or community agreed-upon behavior</li>
    <li>Style merges seamlessly into the surrounding code</li>
    <li>Written as simply and efficiently as possible</li>
    <li>Uses the available PostgreSQL subsystems properly</li>
    <li>Contains sufficient comments</li>
    <li>Contains code that works on all supported operating systems</li>
    <li>Has proper documentation</li>
    <li>Passes all regression tests, and if needed, includes new ones</li>
    <li>Behaves as expected, even under unusual cirumstances</li>
    <li>Contains no reliability risks</li>
    <li>Does not overly complicate the source code</li>
    <li>If performance-related, has a measureable performance benefit</li>
    <li>Is of sufficient usefulness to the average PostgreSQL user</li>
    <li>Follows existing PostgreSQL coding standards</li>
    </ul>

    <H3 id="item1.7">1.7) Where can I learn more about the
    code?</H3>

    <P>Other than documentation in the source tree itself, you can find
    some papers/presentations discussing the code at <A href=
    "http://www.postgresql.org/developer">
    http://www.postgresql.org/developer</A>.  An excellent presentation
    is at <a href=
    "http://neilconway.org/talks/hacking/">http://neilconway.org/talks/hacking/</a></P>

    <H3 id="item1.8">1.8) How do I download/update the current
    source tree?</H3>

    <P>There are several ways to obtain the source tree. Occasional
    developers can just get the most recent source tree snapshot from
    <A href=
    "ftp://ftp.postgresql.org">ftp://ftp.postgresql.org</A>.</P>

    <P>Regular developers might want to take advantage of anonymous
    access to our source code management system. The source tree is
    currently hosted in CVS. For details of how to obtain the source
    from CVS see <A href=
    "http://developer.postgresql.org/docs/postgres/cvs.html">
    http://developer.postgresql.org/docs/postgres/cvs.html</A>.</P>

    <H3 id="item1.9">1.9) How do I test my changes?</H3>

    <P><B>Basic system testing</B></P>

    <P>The easiest way to test your code is to ensure that it builds
    against the latest version of the code and that it does not generate
    compiler warnings.</P>

    <P>It is worth advised that you pass --enable-cassert to
    <I>configure</I>. This will turn on assertions with in the source
    which will often show us bugs because they cause data corruption of
    segmentation violations. This generally makes debugging much
    easier.</P>

    <P>Then, perform run time testing via psql.</P>

    <P><B>Regression test suite</B></P>

    <P>The next step is to test your changes against the existing
    regression test suite. To do this, issue "make check" in the root
    directory of the source tree. If any tests failure,
    investigate.</P>

    <P>If you've deliberately changed existing behavior, this change
    might cause a regression test failure but not any actual regression.
    If so, you should also patch the regression test suite.</P>

    <P><B>Other run time testing</B></P>

    <P>Some developers make use of tools such as valgrind (<A href=
    "http://valgrind.kde.org">http://valgrind.kde.org</A>) for memory
    testing, gprof (which comes with the GNU binutils suite) and
    oprofile (<A href=
    "http://oprofile.sourceforge.net/">http://oprofile.sourceforge.net/</A>)
    for profiling and other related tools.</P>

    <P><B>What about unit testing, static analysis, model
    checking...?</B></P>

    <P>There have been a number of discussions about other testing
    frameworks and some developers are exploring these ideas.</P>

    <P>Keep in mind the <I>Makefiles</I> do not have the proper
    dependencies for include files. You have to do a <I>make clean</I>
    and then another <I>make</I>. If you are using <SMALL>GCC</SMALL>
    you can use the <I>--enable-depend</I> option of <I>configure</I>
    to have the compiler compute the dependencies automatically.</P>

    <H3 id="item1.10">1.10) What tools are available for
    developers?</H3>

    <P>First, all the files in the <I>src/tools</I> directory are
    designed for developers.</P>
<PRE>
    RELEASE_CHANGES changes we have to make for each release
    backend         description/flowchart of the backend directories
    ccsym           find standard defines made by your compiler
     copyright       fixes copyright notices

    entab           converts spaces to tabs, used by pgindent
    find_static     finds functions that could be made static
    find_typedef    finds typedefs in the source code
    find_badmacros  finds macros that use braces incorrectly
    fsync           a script to provide information about the cost of cache
                     syncing system calls
    make_ctags      make vi 'tags' file in each directory
    make_diff       make *.orig and diffs of source
    make_etags      make emacs 'etags' files
    make_keywords   make comparison of our keywords and SQL'92
    make_mkid       make mkid ID files
    pgcvslog        used to generate a list of changes for each release
    pginclude       scripts for adding/removing include files
    pgindent        indents source files
    pgtest          a semi-automated build system
    thread          a thread testing script
</PRE>

    <P>In <I>src/include/catalog</I>:</P>
<PRE>
    unused_oids     a script which generates unused OIDs for use in system
                     catalogs
    duplicate_oids  finds duplicate OIDs in system catalog definitions
</PRE>
    If you point your browser at the <I>tools/backend/index.html</I>
    file, you will see few paragraphs describing the data flow, the
    backend components in a flow chart, and a description of the shared
    memory area. You can click on any flowchart box to see a
    description. If you then click on the directory name, you will be
    taken to the source directory, to browse the actual source code
    behind it. We also have several README files in some source
    directories to describe the function of the module. The browser
    will display these when you enter the directory also. The
    <I>tools/backend</I> directory is also contained on our web page
    under the title <I>How PostgreSQL Processes a Query.</I>

    <P>Second, you really should have an editor that can handle tags,
    so you can tag a function call to see the function definition, and
    then tag inside that function to see an even lower-level function,
    and then back out twice to return to the original function. Most
    editors support this via <I>tags</I> or <I>etags</I> files.</P>

    <P>Third, you need to get <I>id-utils</I> from <A href=
    "ftp://ftp.gnu.org/gnu/id-utils/">ftp://ftp.gnu.org/gnu/id-utils/</A></P>

    <P>By running <I>tools/make_mkid</I>, an archive of source symbols
    can be created that can be rapidly queried.</P>

    <P>Some developers make use of cscope, which can be found at <A
    href="http://cscope.sf.net">http://cscope.sf.net/</A>. Others use
    glimpse, which can be found at <A href=
    "http://webglimpse.net/">http://webglimpse.net/</A>.</P>

    <P><I>tools/make_diff</I> has tools to create patch diff files that
    can be applied to the distribution. This produces context diffs,
    which is our preferred format.</P>

    <P>Our standard format <I>BSD</I> style, with each level of code indented
    one tab, where each tab is four spaces. You will need to set your editor
    or file viewer to display tabs as four spaces:<BR>
    </P>
<PRE>
    vi in ~/.exrc:
            set tabstop=4
            set sw=4
    more:
            more -x4
    less:
            less -x4
</PRE>
    <P>The <I>tools/editors</I> directory of the latest sources contains sample
    settings that can be used with the <I>emacs</I>, <I>xemacs</I> and
    <I>vim</I> editors, that assist in keeping to PostgreSQL coding standards.
    </P>

    <P><I>pgindent</I> will the format code by specifying flags to your
    operating system's utility <I>indent.</I> This <A href=
    "http://ezine.daemonnews.org/200112/single_coding_style.html">article</A>
    describes the value of a consistent coding style.</P>

    <P><I>pgindent</I> is run on all source files just before each beta
    test period. It auto-formats all source files to make them
    consistent. Comment blocks that need specific line breaks should be
    formatted as <I>block comments,</I> where the comment starts as
    <CODE>/*------</CODE>. These comments will not be reformatted in
    any way.</P>

    <P><I>pginclude</I> contains scripts used to add needed
    <CODE>#include</CODE>'s to include files, and removed unneeded
    <CODE>#include</CODE>'s.</P>

    <P>When adding system types, you will need to assign oids to them.
    There is also a script called <I>unused_oids</I> in
    <I>pgsql/src/include/catalog</I> that shows the unused oids.</P>

    <H3 id="item1.11">1.11) What books are good for
    developers?</H3>

    <P>I have four good books, <I>An Introduction to Database
    Systems,</I> by C.J. Date, Addison, Wesley, <I>A Guide to the SQL
    Standard,</I> by C.J. Date, et. al, Addison, Wesley,
    <I>Fundamentals of Database Systems,</I> by Elmasri and Navathe,
    and <I>Transaction Processing,</I> by Jim Gray, Morgan,
    Kaufmann</P>

    <P>There is also a database performance site, with a handbook
    on-line written by Jim Gray at <A href=
    "http://www.benchmarkresources.com">http://www.benchmarkresources.com.</A>.</P>

    <H3 id="item1.12">1.12) What is configure all about?</H3>

    <P>The files <I>configure</I> and <I>configure.in</I> are part of
    the GNU <I>autoconf</I> package. Configure allows us to test for
    various capabilities of the OS, and to set variables that can then
    be tested in C programs and Makefiles. Autoconf is installed on the
    PostgreSQL main server. To add options to configure, edit
    <I>configure.in,</I> and then run <I>autoconf</I> to generate
    <I>configure.</I></P>

    <P>When <I>configure</I> is run by the user, it tests various OS
    capabilities, stores those in <I>config.status</I> and
    <I>config.cache,</I> and modifies a list of <I>*.in</I> files. For
    example, if there exists a <I>Makefile.in,</I> configure generates
    a <I>Makefile</I> that contains substitutions for all @var@
    parameters found by configure.</P>

    <P>When you need to edit files, make sure you don't waste time
    modifying files generated by <I>configure.</I> Edit the <I>*.in</I>
    file, and re-run <I>configure</I> to recreate the needed file. If
    you run <I>make distclean</I> from the top-level source directory,
    all files derived by configure are removed, so you see only the
    file contained in the source distribution.</P>

    <H3 id="item1.13">1.13) How do I add a new port?</H3>

    <P>There are a variety of places that need to be modified to add a
    new port. First, start in the <I>src/template</I> directory. Add an
    appropriate entry for your OS. Also, use <I>src/config.guess</I> to
    add your OS to <I>src/template/.similar.</I> You shouldn't match
    the OS version exactly. The <I>configure</I> test will look for an
    exact OS version number, and if not found, find a match without
    version number. Edit <I>src/configure.in</I> to add your new OS.
    (See configure item above.) You will need to run autoconf, or patch
    <I>src/configure</I> too.</P>

    <P>Then, check <I>src/include/port</I> and add your new OS file,
    with appropriate values. Hopefully, there is already locking code
    in <I>src/include/storage/s_lock.h</I> for your CPU. There is also
    a <I>src/makefiles</I> directory for port-specific Makefile
    handling. There is a <I>backend/port</I> directory if you need
    special files for your OS.</P>

    <H3 id="item1.14">1.14) Why don't you use threads, raw
    devices, async-I/O, &lt;insert your favorite wizz-bang feature
    here&gt;?</H3>

    <P>There is always a temptation to use the newest operating system
    features as soon as they arrive. We resist that temptation.</P>

    <P>First, we support 15+ operating systems, so any new feature has
    to be well established before we will consider it. Second, most new
    <I>wizz-bang</I> features don't provide <I>dramatic</I>
    improvements. Third, they usually have some downside, such as
    decreased reliability or additional code required. Therefore, we
    don't rush to use new features but rather wait for the feature to
    be established, then ask for testing to show that a measurable
    improvement is possible.</P>

    <P>As an example, threads are not currently used in the backend
    code because:</P>

    <UL>
      <LI>Historically, threads were unsupported and buggy.</LI>

      <LI>An error in one backend can corrupt other backends.</LI>

      <LI>Speed improvements using threads are small compared to the
      remaining backend startup time.</LI>

      <LI>The backend code would be more complex.</LI>
    </UL>

    <P>So, we are not ignorant of new features. It is just that we are
    cautious about their adoption. The TODO list often contains links
    to discussions showing our reasoning in these areas.</P>

    <H3 id="item1.15">1.15) How are RPMs packaged?</H3>

    <P>This was written by Lamar Owen and Devrim Gündüz:</P>

    <P>2006-10-16</P>

    <P>
   As to how the RPMs are built -- to answer that question sanely
   requires us to know how much experience you have with the whole RPM
   paradigm. 'How is the RPM built?' is a multifaceted question. The
   obvious simple answer is that we maintain:</P>
    <OL>
      <LI>A set of patches to make certain portions of the source tree
      'behave' in the different environment of the RPMset;</LI>

      <LI>The initscript;</LI>

      <LI>Any other ancillary scripts and files;</LI>

      <LI>A README.rpm-dist document that tries to adequately document
      both the differences between the RPM build and the WHY of the
      differences, as well as useful RPM environment operations (like,
      using syslog, upgrading, getting postmaster to start at OS boot,
      etc);</LI>

      <LI>The spec file that throws it all together. This is not a
      trivial undertaking in a package of this size.</LI>
    </OL>

   <P>PGDG RPM Maintainer builds the SRPM and announces the SRPM to the
   pgsqlrpms-hackers list. This is a list where package builders are
   subscribed. Then, the builders download the SRPM and rebuild it on their
   machines.</P>

   <P>We try to build on as many different canonical distributions as we can.
   Currently we are able to build on Red Hat Linux 9, RHEL 3 and above,
   and all Fedora Core Linux releases.</P>

   <P>To test the binaries, we install them on our local machines and run
   regression tests. If the package builders uses postgres user to build the
   rpms, then it is possible to run regression tests during RPM builds.</P>

   <P>Once the build passes these tests, the binary RPMs are sent back to PGDG
   RPM Maintainer and they are pushed to main FTP site, followed by a
   release announcement to pgsqlrpms-* lists, pgsql-general and
   pgsql-announce lists.</P>

   <P>You will notice we said 'canonical' distributions above. That simply
   means that the machine is as stock 'out of the box' as practical --
   that is, everything (except select few programs) on these boxen are
   installed by RPM; only official Red Hat released RPMs are used (except
   in unusual circumstances involving software that will not alter the
   build -- for example, installing a newer non-RedHat version of the Dia
   diagramming package is OK -- installing Python 2.1 on the box that has
   Python 1.5.2 installed is not, as that alters the PostgreSQL build).
   The RPM as uploaded is built to as close to out-of-the-box pristine as
   is possible. Only the standard released 'official to that release'
   compiler is used -- and only the standard official kernel is used as
   well.</P>

   <P>PGDG RPM Building Project does not build RPMs for Mandrake .</P>

   <P>We usually have only one SRPM for all platforms. This is because of our
   limited resources. However, on some cases, we may distribute different
   SRPMs for different platforms, depending on possible compilation problems,
   especially on older distros.</P>

   <P>Please note that this is a volunteered job -- We are doing our best to
   keep  packages up to date. We, at least, provide SRPMs for all platforms.
   For example, if you do not find a RHEL 4 x86_64 RPM in our FTP site, it
   means that we do not have a RHEL 4 x86_64 server around. If you have one
   and want to help us, please do not hesitate to build rpms and send to us :-)
   http://pgfoundry.org/docman/view.php/1000048/98/PostgreSQL-RPM-Installation-PGDG.pdf
   has some information about building binary RPMs using an SRPM.</P>

   <P>PGDG RPM Building Project is a hosted on pgFoundry :
   <a href="http://pgfoundry.org/projects/pgsqlrpms">http://pgfoundry.org/projects/pgsqlrpms</a>.
   We are an open community, except one point : Our pgsqlrpms-hackers list is open
   to package builders only. Still, its archives are visible to public.
   We use a CVS server to save  the work we have done so far. This includes
   spec files and patches; as well as documents.</P>

   <P>As to why all these files aren't part of the source tree, well, unless
   there was a large cry for it to happen, we don't believe it should.</P>

    <H3 id="item1.16">1.16) How are CVS branches managed?</H3>

    <P>This was written by Tom Lane:</P>

    <P>2001-05-07</P>

    <P>If you just do basic "cvs checkout", "cvs update", "cvs commit",
    then you'll always be dealing with the HEAD version of the files in
    CVS. That's what you want for development, but if you need to patch
    past stable releases then you have to be able to access and update
    the "branch" portions of our CVS repository. We normally fork off a
    branch for a stable release just before starting the development
    cycle for the next release.</P>

    <P>The first thing you have to know is the branch name for the
    branch you are interested in getting at. To do this, look at some
    long-lived file, say the top-level HISTORY file, with "cvs status
    -v" to see what the branch names are. (Thanks to Ian Lance Taylor
    for pointing out that this is the easiest way to do it.) Typical
    branch names are:</P>
<PRE>
    REL7_1_STABLE
    REL7_0_PATCHES
    REL6_5_PATCHES
</PRE>

    <P>OK, so how do you do work on a branch? By far the best way is to
    create a separate checkout tree for the branch and do your work in
    that. Not only is that the easiest way to deal with CVS, but you
    really need to have the whole past tree available anyway to test
    your work. (And you *better* test your work. Never forget that
    dot-releases tend to go out with very little beta testing --- so
    whenever you commit an update to a stable branch, you'd better be
    doubly sure that it's correct.)</P>

    <P>Normally, to checkout the head branch, you just cd to the place
    you want to contain the toplevel "pgsql" directory and say</P>
<PRE>
    cvs ... checkout pgsql
</PRE>

    <P>To get a past branch, you cd to wherever you want it and
    say</P>
<PRE>
    cvs ... checkout -r BRANCHNAME pgsql
</PRE>

    <P>For example, just a couple days ago I did</P>
<PRE>
    mkdir ~postgres/REL7_1
    cd ~postgres/REL7_1
    cvs ... checkout -r REL7_1_STABLE pgsql
</PRE>

    <P>and now I have a maintenance copy of 7.1.*.</P>

    <P>When you've done a checkout in this way, the branch name is
    "sticky": CVS automatically knows that this directory tree is for
    the branch, and whenever you do "cvs update" or "cvs commit" in
    this tree, you'll fetch or store the latest version in the branch,
    not the head version. Easy as can be.</P>

    <P>So, if you have a patch that needs to apply to both the head and
    a recent stable branch, you have to make the edits and do the
    commit twice, once in your development tree and once in your stable
    branch tree. This is kind of a pain, which is why we don't normally
    fork the tree right away after a major release --- we wait for a
    dot-release or two, so that we won't have to double-patch the first
    wave of fixes.</P>

    <H3 id="item1.17">1.17) Where can I get a copy of the SQL
    standards?</H3>

    <P>There are three versions of the SQL standard: SQL-92, SQL:1999,
    and SQL:2003. They are endorsed by ANSI and ISO. Draft versions can
    be downloaded from:</P>

    <UL>
      <LI>SQL-92 <A href=
      "http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt">http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt</A></LI>

      <LI>SQL:1999 <A href=
      "http://www.cse.iitb.ac.in/dbms/Data/Papers-Other/SQL1999/ansi-iso-9075-2-1999.pdf">
      http://www.cse.iitb.ac.in/dbms/Data/Papers-Other/SQL1999/ansi-iso-9075-2-1999.pdf</A></LI>

      <LI>SQL:2003 <A href=
      "http://www.wiscorp.com/sql_2003_standard.zip">http://www.wiscorp.com/sql_2003_standard.zip</A></LI>
    </UL>

    <P>Some SQL standards web pages are:</P>

    <UL>
      <LI><A href=
      "http://troels.arvin.dk/db/rdbms/links/#standards">http://troels.arvin.dk/db/rdbms/links/#standards</A></LI>

      <LI><A href=
      "http://www.wiscorp.com/SQLStandards.html">http://www.wiscorp.com/SQLStandards.html</A></LI>

      <LI><A href=
      "http://www.contrib.andrew.cmu.edu/~shadow/sql.html#syntax">http://www.contrib.andrew.cmu.edu/~shadow/sql.html#syntax</A>
      (SQL-92)</LI>

      <LI><A href=
      "http://dbs.uni-leipzig.de/en/lokal/standards.pdf">http://dbs.uni-leipzig.de/en/lokal/standards.pdf</A>
      (paper)</LI>
    </UL>

    <H3 id="item1.18">1.18) Where can I get technical
    assistance?</H3>

    <P>Many technical questions held by those new to the code have been
    answered on the pgsql-hackers mailing list - the archives of which
    can be found at <A href=
    "http://archives.postgresql.org/pgsql-hackers/">http://archives.postgresql.org/pgsql-hackers/</A>.</P>

    <P>If you cannot find discussion or your particular question, feel
    free to put it to the list.</P>

    <P>Major contributors also answer technical questions, including
    questions about development of new features, on IRC at
    irc.freenode.net in the #postgresql channel.</P>

    <H3 id="item1.19">1.19) How do I get involved in PostgreSQL
    web site development?</H3>

    <P>PostgreSQL website development is discussed on the
    pgsql-www@postgresql.org mailing list. The is a project page where
    the source code is available at <A href=
    "http://gborg.postgresql.org/project/pgweb/projdisplay.php">http://gborg.postgresql.org/project/pgweb/projdisplay.php</A>
    , the code for the next version of the website is under the
    "portal" module.</P>

    <H3 id="item1.20">1.20) Why haven't you replaced CVS with SVN, Git,
    Monotone, VSS, &lt;insert your favorite SCMS here&gt;?</H3>

    <P>Currently the core developers see no SCMS that will provide
    enough benefit to outwiegh the pain involved in moving to a new
    SCMS. Typical problems that must be addressed by any new SCMS include:</P>

    <ul>
    <li>Run natively on all of our <a href="http://www.postgresql.org/docs/current/interactive/supported-platforms.html">supported platforms</a>.</li>
    <li>Integrate into the <a href="http://pgbuildfarm.org/">Buildfarm</a>.</li>
    <li>Import our entire CVS Repository while preserving complete history.</li>
    <li>Allow for anonymous checkouts.</li>
    </ul>

    <P>Currently there is no intention for switching to a new SCMS until at least the
    end of the 8.4 development cycle sometime in late 2008. For more information
    please refer to the mailing list archives.</P>


      <H2>Technical Questions</H2>

    <H3 id="item2.1">2.1) How do I efficiently access information
    in tables from the backend code?</H3>

    <P>You first need to find the tuples(rows) you are interested in.
    There are two ways. First, <I>SearchSysCache()</I> and related
    functions allow you to query the system catalogs. This is the
    preferred way to access system tables, because the first call to
    the cache loads the needed rows, and future requests can return the
    results without accessing the base table. The caches use system
    table indexes to look up tuples. A list of available caches is
    located in <I>src/backend/utils/cache/syscache.c.</I>
    <I>src/backend/utils/cache/lsyscache.c</I> contains many
    column-specific cache lookup functions.</P>

    <P>The rows returned are cache-owned versions of the heap rows.
    Therefore, you must not modify or delete the tuple returned by
    <I>SearchSysCache()</I>. What you <I>should</I> do is release it
    with <I>ReleaseSysCache()</I> when you are done using it; this
    informs the cache that it can discard that tuple if necessary. If
    you neglect to call <I>ReleaseSysCache()</I>, then the cache entry
    will remain locked in the cache until end of transaction, which is
    tolerable but not very desirable.</P>

    <P>If you can't use the system cache, you will need to retrieve the
    data directly from the heap table, using the buffer cache that is
    shared by all backends. The backend automatically takes care of
    loading the rows into the buffer cache.</P>

    <P>Open the table with <I>heap_open().</I> You can then start a
    table scan with <I>heap_beginscan(),</I> then use
    <I>heap_getnext()</I> and continue as long as
    <I>HeapTupleIsValid()</I> returns true. Then do a
    <I>heap_endscan().</I> <I>Keys</I> can be assigned to the
    <I>scan.</I> No indexes are used, so all rows are going to be
    compared to the keys, and only the valid rows returned.</P>

    <P>You can also use <I>heap_fetch()</I> to fetch rows by block
    number/offset. While scans automatically lock/unlock rows from the
    buffer cache, with <I>heap_fetch(),</I> you must pass a
    <I>Buffer</I> pointer, and <I>ReleaseBuffer()</I> it when
    completed.</P>

    <P>Once you have the row, you can get data that is common to all
    tuples, like <I>t_self</I> and <I>t_oid,</I> by merely accessing
    the <I>HeapTuple</I> structure entries. If you need a
    table-specific column, you should take the HeapTuple pointer, and
    use the <I>GETSTRUCT()</I> macro to access the table-specific start
    of the tuple. You then cast the pointer as a <I>Form_pg_proc</I>
    pointer if you are accessing the pg_proc table, or
    <I>Form_pg_type</I> if you are accessing pg_type. You can then
    access the columns by using a structure pointer:</P>
<PRE>
<CODE>((Form_pg_class) GETSTRUCT(tuple))-&gt;relnatts
</CODE>
</PRE>
    You must not directly change <I>live</I> tuples in this way. The
    best way is to use <I>heap_modifytuple()</I> and pass it your
    original tuple, and the values you want changed. It returns a
    palloc'ed tuple, which you pass to <I>heap_replace().</I> You can
    delete tuples by passing the tuple's <I>t_self</I> to
    <I>heap_destroy().</I> You use <I>t_self</I> for
    <I>heap_update()</I> too. Remember, tuples can be either system
    cache copies, which might go away after you call
    <I>ReleaseSysCache()</I>, or read directly from disk buffers, which
    go away when you <I>heap_getnext()</I>, <I>heap_endscan</I>, or
    <I>ReleaseBuffer()</I>, in the <I>heap_fetch()</I> case. Or it may
    be a palloc'ed tuple, that you must <I>pfree()</I> when finished.

    <H3 id="item2.2">2.2) Why are table, column, type, function,
    view names sometimes referenced as <I>Name</I> or <I>NameData,</I>
    and sometimes as <I>char *?</I></H3>

    <P>Table, column, type, function, and view names are stored in
    system tables in columns of type <I>Name.</I> Name is a
    fixed-length, null-terminated type of <I>NAMEDATALEN</I> bytes.
    (The default value for NAMEDATALEN is 64 bytes.)</P>
<PRE>
<CODE>typedef struct nameData
    {
        char        data[NAMEDATALEN];
    } NameData;
    typedef NameData *Name;
</CODE>
</PRE>
    Table, column, type, function, and view names that come into the
    backend via user queries are stored as variable-length,
    null-terminated character strings.

    <P>Many functions are called with both types of names, ie.
    <I>heap_open().</I> Because the Name type is null-terminated, it is
    safe to pass it to a function expecting a char *. Because there are
    many cases where on-disk names(Name) are compared to user-supplied
    names(char *), there are many cases where Name and char * are used
    interchangeably.</P>

    <H3 id="item2.3">2.3) Why do we use <I>Node</I> and
    <I>List</I> to make data structures?</H3>

    <P>We do this because this allows a consistent way to pass data
    inside the backend in a flexible way. Every node has a
    <I>NodeTag</I> which specifies what type of data is inside the
    Node. <I>Lists</I> are groups of <I>Nodes chained together as a
    forward-linked list.</I></P>

    <P>Here are some of the <I>List</I> manipulation commands:</P>

    <BLOCKQUOTE>
      <DL>
        <DT>lfirst(i), lfirst_int(i), lfirst_oid(i)</DT>

        <DD>return the data (a pointer, integer or OID respectively) of
        list cell <I>i.</I></DD>

        <DT>lnext(i)</DT>

        <DD>return the next list cell after <I>i.</I></DD>

        <DT>foreach(i, list)</DT>

        <DD>
          loop through <I>list,</I> assigning each list cell to
          <I>i.</I> It is important to note that <I>i</I> is a ListCell *,
          not the data in the <I>List</I> element. You need to use
          <I>lfirst(i)</I> to get at the data. Here is a typical code
          snippet that loops through a List containing <I>Var *'s</I>
          and processes each one:
<PRE>
<CODE>
    List        *list;
    ListCell    *i;

    foreach(i, list)
    {
        Var *var = lfirst(i);

        /* process var here */
    }
</CODE>
</PRE>
        </DD>

        <DT>lcons(node, list)</DT>

        <DD>add <I>node</I> to the front of <I>list,</I> or create a
        new list with <I>node</I> if <I>list</I> is <I>NIL.</I></DD>

        <DT>lappend(list, node)</DT>

        <DD>add <I>node</I> to the end of <I>list.</I></DD>

        <DT>list_concat(list1, list2)</DT>

        <DD>Concatenate <I>list2</I> on to the end of <I>list1.</I></DD>

        <DT>list_length(list)</DT>

        <DD>return the length of the <I>list.</I></DD>

        <DT>list_nth(list, i)</DT>

        <DD>return the <I>i</I>'th element in <I>list,</I>
         counting from zero.</DD>

        <DT>lcons_int, ...</DT>

        <DD>There are integer versions of these: <I>lcons_int,
        lappend_int</I>, etc. Also versions for OID lists: <I>lcons_oid,
        lappend_oid</I>, etc.</DD>
      </DL>
    </BLOCKQUOTE>
    You can print nodes easily inside <I>gdb.</I> First, to disable
    output truncation when you use the gdb <I>print</I> command:
<PRE>
<CODE>(gdb) set print elements 0
</CODE>
</PRE>
    Instead of printing values in gdb format, you can use the next two
    commands to print out List, Node, and structure contents in a
    verbose format that is easier to understand. List's are unrolled
    into nodes, and nodes are printed in detail. The first prints in a
    short format, and the second in a long format:
<PRE>
<CODE>(gdb) call print(any_pointer)
    (gdb) call pprint(any_pointer)
</CODE>
</PRE>
    The output appears in the postmaster log file, or on your screen if
    you are running a backend directly without a postmaster.

    <H3 id="item2.4">2.4) I just added a field to a structure.
    What else should I do?</H3>

    <P>The structures passed around in the parser, rewriter,
    optimizer, and executor require quite a bit of support. Most
    structures have support routines in <I>src/backend/nodes</I> used
    to create, copy, read, and output those structures (in particular,
    the files <I>copyfuncs.c</I> and <I>equalfuncs.c</I>. Make sure you
    add support for your new field to these files. Find any other
    places the structure might need code for your new field. <I>mkid</I>
    is helpful with this (see <A href="#item1.10">1.10</A>).</P>

    <H3 id="item2.5">2.5) Why do we use <I>palloc</I>() and
    <I>pfree</I>() to allocate memory?</H3>

    <P><I>palloc()</I> and <I>pfree()</I> are used in place of malloc()
    and free() because we find it easier to automatically free all
    memory allocated when a query completes. This assures us that all
    memory that was allocated gets freed even if we have lost track of
    where we allocated it. There are special non-query contexts that
    memory can be allocated in. These affect when the allocated memory
    is freed by the backend.</P>

    <H3 id="item2.6">2.6) What is ereport()?</H3>

    <P><I>ereport()</I> is used to send messages to the front-end, and
    optionally terminate the current query being processed. The first
    parameter is an ereport level of <I>DEBUG</I> (levels 1-5),
    <I>LOG,</I> <I>INFO,</I> <I>NOTICE,</I> <I>ERROR,</I> <I>FATAL,</I>
    or <I>PANIC.</I> <I>NOTICE</I> prints on the user's terminal and
    the postmaster logs. <I>INFO</I> prints only to the user's terminal
    and <I>LOG</I> prints only to the server logs. (These can be
    changed from <I>postgresql.conf.</I>) <I>ERROR</I> prints in both
    places, and terminates the current query, never returning from the
    call. <I>FATAL</I> terminates the backend process. The remaining
    parameters of <I>ereport</I> are a <I>printf</I>-style set of
    parameters to print.</P>

    <P><I>ereport(ERROR)</I> frees most memory and open file
    descriptors so you don't need to clean these up before the
    call.</P>

    <H3 id="item2.7">2.7) What is CommandCounterIncrement()?</H3>

    <P>Normally, transactions can not see the rows they modify.
    This allows <CODE>UPDATE foo SET x = x + 1</CODE> to work
    correctly.</P>

    <P>However, there are cases where a transactions needs to see
    rows affected in previous parts of the transaction. This is
    accomplished using a Command Counter. Incrementing the counter
    allows transactions to be broken into pieces so each piece can
    see rows modified by previous pieces. <I>CommandCounterIncrement()</I>
    increments the Command Counter, creating a new part of the
    transaction.</P>

    <H3 id="item2.8">2.8) What debugging features are available?</H3>

    <P>First, try running <I>configure</I> with the --enable-cassert
    option, many <I>assert()</I>s monitor the progress of the
    backend and halt the program when something unexpected occurs.</P>

    <P>The <I>postmaster</I> has a <I>-d</I> option that allows
    even more detailed information to be reported. The <I>-d</I>
    option takes a number that specifies the debug level. Be warned
    that high debug level values generate large log files.</P>

    <P>If the <I>postmaster</I> is not running, you can actually
    run the <I>postgres</I> backend from the command line, and type
    your <SMALL>SQL</SMALL> statement directly. This is recommended
    <B>only</B> for debugging purposes. If you have compiled with
    debugging symbols, you can use a debugger to see what is
    happening. Because the backend was not started from <I>postmaster</I>,
    it is not running in an identical environment and locking/backend
    interaction problems might not be duplicated.</P>

    <P>If the <I>postmaster</I> is running, start <I>psql</I> in
    one window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
    process used by <I>psql</I> using <CODE>SELECT pg_backend_pid()</CODE>.
    Use a debugger to attach to the <I>postgres</I> <SMALL>PID</SMALL>.
    You can set breakpoints in the debugger and issue queries from
    the other.  If you are looking to find the location that is
    generating an error or log message, set a breakpoint at
    <I>errfinish</I>.

    <I>psql</I>. If you are debugging <I>postgres</I> startup, you
    can set PGOPTIONS="-W n", then start <I>psql</I>. This will
    cause startup to delay for <I>n</I> seconds so you can attach
    to the process with the debugger, set any breakpoints, and
    continue through the startup sequence.</P>

    <P>You can also compile with profiling to see what functions
    are taking execution time. The backend profile files will be
    deposited in the <I>pgsql/data</I> directory. The client profile
    file will be put in the client's current directory. Linux
    requires a compile with <I>-DLINUX_PROFILE</I> for proper
    profiling.</P>

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