Developer's Frequently Asked Questions (FAQ) for PostgreSQL

Last updated: Wed Feb 11 20:23:01 EST 1998

Current maintainer: Bruce Momjian (maillist@candle.pha.pa.us)

The most recent version of this document can be viewed at the postgreSQL Web
site, http://postgreSQL.org.

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Questions answered:

1) What tools are available for developers?
2) What books are good for developers?
3) Why do we use palloc() and pfree() to allocate memory?
4) Why do we use Node and List to make data structures?
5) How do I add a feature or fix a bug?
6) How do I download/update the current source tree?
7) How do I test my changes?

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1) What tools are available for developers?

Aside from the User documentation mentioned in the regular FAQ, there
are several development tools available. First, all the files in the
pgsql/src/tools directory are designed for developers.

        RELEASE_CHANGES         changes we have to make for each release
        SQL_keywords            standard SQL'92 keywords
        backend                 web flowchart of the backend directories
        ccsym                   find standard defines made by your compiler
        entab                   converts tabs to spaces, used by pgindent
        find_static             finds functions that could be made static
        find_typedef            get a list of typedefs in the source code
        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.README    make comparison of our keywords and SQL'92
        make_mkid               make mkid ID files
        mkldexport              create AIX exports file
        pgindent                indents C source files

Let me note some of these. If you point your browser at the
pgsql/src/tools/backend directory, you will see all the backend
components in a flow chart. You can click on any one 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 pgsql/src/tools/backend directory is also
contained on our web page under the title Backend Flowchart.

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 tags or etags
files.

Third, you need to get mkid from ftp.postgresql.org. By running
tools/make_mkid, an archive of source symbols can be created that can be
rapidly queried like grep or edited.

make_diff has tools to create patch diff files that can be applied to the
distribution.

pgindent will format source files to match our standard format, which has
four-space tabs, and an indenting format specified by flags to the your
operating system's utility indent.

2) What books are good for developers?

I have three good books, An Introduction to Database Systems, by C.J. Date,
Addison, Wesley, A Guide to the SQL Standard, by C.J. Date, et. al,
Addison, Wesley, and Transaction Processing:  Concepts and Techniques,
by Jim Gray and Andreas Reuter, Morgan, Kaufmann.

3) Why do we use palloc() and pfree() to allocate memory?

palloc() and pfree() are used in place of malloc() and free() because we
automatically free all memory allocated when a transaction completes. This
makes it easier to make sure we free memory that gets allocated in one
place, but only freed much later. There are several contexts that memory can
be allocated in, and this controls when the allocated memory is
automatically freed by the backend.

4) Why do we use Node and List to make data structures?

We do this because this allows a consistent way to pass data inside the
backend in a flexible way. Every node has a NodeTag which specifies what
type of data is inside the Node. Lists are lists of Nodes. lfirst(),
lnext(), and foreach() are used to get, skip, and traverse through Lists.

5) How do I add a feature or fix a bug?

The source code is over 250,000 lines. Many problems/features are isolated
to one specific area of the code. Others require knowledge of much of the
source. If you are confused about where to start, ask the hackers list, and
they will be glad to assess the complexity and give pointers on where to
start.

Another thing to keep in mind is that many fixes and features can be added
with surprisingly little code. I often start by adding code, then looking at
other areas in the code where similar things are done, and by the time I am
finished, the patch is quite small and compact.

When adding code, keep in mind that it should use the existing facilities in
the source, for performance reasons and for simplicity. Often a review of
existing code doing similar things is helpful.

6) How do I download/update the current source tree?

There are several ways to obtain the source tree. Occasional developers can
just get the most recent source tree snapshot from ftp.postgresql.org. For
regular developers, you can get CVSup, which is available from
ftp.postgresql.org too. CVSup allows you to download the source tree, then
occasionally update your copy of the source tree with any new changes. Using
CVSup, you don't have to download the entire source each time, only the
changed files. CVSup does not allow developers to update the source tree.

To update the source tree, there are two ways. You can generate a patch
against your current source tree, perhaps using the make_diff tools
mentioned above, and send them to the patches list. They will be reviewed,
and applied in a timely manner. If the patch is major, and we are in beta
testing, the developers may wait for the final release before applying your
patches.

For hard-core developers, Marc(scrappy@postgresql.org) will give you a Unix
shell account on postgresql.org, and you can ftp your files into your
account, patch, and cvs install the changes directly into the source tree.

6) How do I test my changes?

First, use psql to make sure it is working as you expect. Then run
src/test/regress and get the output of src/test/regress/checkresults with
and without your changes, to see that your patch does not change the
regression test in unexpected ways. This practice has saved me many times.
The regression tests test the code in ways I would never do, and has caught
many bugs in my patches. By finding the problems now, you save yourself a
lot of debugging later when things are broken, and you can't figure out when
it happened.
