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<TITLE>How PostgreSQL Processes a Query</TITLE>
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<H1>
How PostgreSQL Processes a Query
</H1>
<H2>
by Bruce Momjian
</H2>
<P>
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<EM>
Click on an item to see more detail or look at the full
<A HREF="backend_dirs.html">index.</A>
</EM>
<BR>
<BR>
</P>
<P>
A query comes to the backend via data packets arriving through TCP/IP or
Unix Domain sockets. It is loaded into a string, and passed to the
<A HREF="../../backend/parser">parser,</A> where the lexical scanner,
<A HREF="../../backend/parser/scan.l">scan.l,</A> breaks the query up
into tokens(words). The parser uses <A
HREF="../../backend/parser/gram.y">gram.y</A> and the tokens to identify
the query type, and load the proper query-specific structure, like <A
HREF="../../include/nodes/parsenodes.h">CreateStmt</A> or <A
HREF="../../include/nodes/parsenodes.h">SelectStmt.</A></P><P>
The query is then identified as a <I>Utility</I> query or a more complex
query. A <I>Utility</I> query is processed by a query-specific function
in <A HREF="../../backend/commands"> commands.</A> A complex query, like
<I>SELECT, UPDATE,</I> and <I>DELETE</I> requires much more handling.</P><P>
The parser takes a complex query, and creates a
<A HREF="../../include/nodes/parsenodes.h">Query</A> structure that
contains all the elements used by complex queries. Query.qual holds the
<I>WHERE</I> clause qualification, which is filled in by <A
HREF="../../backend/parser/parse_clause.c">transformWhereClause().</A>
Each table referenced in the query is represented by a <A
HREF="../../include/nodes/parsenodes.h"> RangeTableEntry,</A> and they
are linked together to form the <I>range table</I> of the query, which
is generated by <A HREF="../../backend/parser/parse_clause.c">
transformFromClause().</A> Query.rtable holds the query's range table.</P><P>
Certain queries, like <I>SELECT,</I> return columns of data. Other
queries, like <I>INSERT</I> and <I>UPDATE,</I> specify the columns
modified by the query. These column references are converted to <A
HREF="../../include/nodes/primnodes.h">TargetEntry</A> entries, which are
linked together to make up the <I>target list</I> of
the query. The target list is stored in Query.targetList, which is
generated by <A
HREF="../../backend/parser/parse_target.c">transformTargetList().</A></P><P>
Other query elements, like aggregates(<I>SUM()</I>), <I>GROUP BY,</I>
and <I>ORDER BY</I> are also stored in their own Query fields.</P><P>
The next step is for the Query to be modified by any <I>VIEWS</I> or
<I>RULES</I> that may apply to the query. This is performed by the <A
HREF="../../backend/rewrite">rewrite</A> system.</P><P>
The <A HREF="../../backend/optimizer">optimizer</A> takes the Query
structure and generates an optimal <A
HREF="../../include/nodes/plannodes.h">Plan,</A> which contains the
operations to be performed to execute the query. The <A
HREF="../../backend/optimizer/path">path</A> module determines the best
table join order and join type of each table in the RangeTable, using
Query.qual(<I>WHERE</I> clause) to consider optimal index usage.</P><P>
The Plan is then passed to the <A
HREF="../../backend/executor">executor</A> for execution, and the result
returned to the client. The Plan actually as set of nodes, arranged in
a tree structure with a top-level node, and various sub-nodes as
children.</P><P>
There are many other modules that support this basic functionality. They
can be accessed by clicking on the flowchart.</P>
<HR><P>
Another area of interest is the shared memory area, which contains data
accessable to all backends. It has recently used data/index blocks,
locks, backend process information, and lookup tables for these
structures:
</P>
<UL>
<LI>ShmemIndex - lookup shared memory addresses using structure names</LI>
<LI><A HREF="../../include/storage/buf_internals.h">Buffer
Descriptor</A> - control header for buffer cache block</LI>
<LI><A HREF="../../include/storage/buf_internals.h">Buffer Block</A> -
data/index buffer cache block</LI>
<LI>Shared Buffer Lookup Table - lookup of buffer cache block addresses
using table name and block number(<A
HREF="../../include/storage/buf_internals.h"> BufferTag</A>)</LI>
<LI>MultiLevelLockTable (ctl) - control structure for each locking
method. Currently, only multi-level locking is used(<A
HREF="../../include/storage/lock.h">LOCKMETHODCTL</A>).</LI>
<LI>MultiLevelLockTable (lock hash) - the <A
HREF="../../include/storage/lock.h">LOCK</A> structure, looked up using
relation, database object ids(<A
HREF="../../include/storage/lock.h">LOCKTAG)</A>. The lock table
structure contains the lock modes(read/write or shared/exclusive) and
circular linked list of backends (<A
HREF="../../include/storage/proc.h">PROC</A> structure pointers) waiting
on the lock.</LI>
<LI>MultiLevelLockTable (xid hash) - lookup of LOCK structure address
using transaction id, LOCK address. It is used to quickly check if the
current transaction already has any locks on a table, rather than having
to search through all the held locks. It also stores the modes
(read/write) of the locks held by the current transaction. The returned
<A HREF="../../include/storage/lock.h">XIDLookupEnt</A> structure also
contains a pointer to the backend's PROC.lockQueue.</LI>
<LI><A HREF="../../include/storage/proc.h">Proc Header</A> - information
about each backend, including locks held/waiting, indexed by process id</LI>
</UL>
<P>Each data structure is created by calling <A
HREF="../../backend/storage/ipc/shmem.c">ShmemInitStruct(),</A> and the
lookups are created by <A
HREF="../../backend/storage/ipc/shmem.c">ShmemInitHash().</A></P>
<HR>
<SMALL>
Maintainer: Bruce Momjian (<A
HREF="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
Last updated: Mon Aug 10 10:48:06 EDT 1998
</SMALL>
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<title>How PostgreSQL Processes a Query</title>
</head>
<body bgcolor="#FFFFFF" text="#000000" link="#FF0000"
vlink="#A00000" alink="#0000FF">
<h1>How PostgreSQL Processes a Query</h1>
<h2>by Bruce Momjian</h2>
<p><img src="flow.gif" usemap="#flowmap" alt="flowchart" />
<em>Click on an item to see more detail or look at the full
<a href="backend_dirs.html">index.</a></em>
<map name="flowmap" id="flowmap">
<area coords="125,35,245,65" href="backend_dirs.html#main" alt="main" />
<area coords="125,100,245,125" href="backend_dirs.html#postmaster" alt="postmaster" />
<area coords="325,65,450,95" href="backend_dirs.html#libpq" alt="libpq" />
<area coords="125,160,245,190" href="backend_dirs.html#tcop" alt="tcop" />
<area coords="325,160,450,190" href="backend_dirs.html#tcop" alt="tcop" />
<area coords="125,240,245,265" href="backend_dirs.html#parser" alt="parser" />
<area coords="125,300,250,330" href="backend_dirs.html#tcop" alt="tcop" />
<area coords="125,360,250,390" href="backend_dirs.html#optimizer" alt="optimizer" />
<area coords="125,425,245,455" href="backend_dirs.html#optimizer_plan" alt="plan" />
<area coords="125,490,245,515" href="backend_dirs.html#executor" alt="executor" />
<area coords="325,300,450,330" href="backend_dirs.html#commands" alt="commands" />
<area coords="75,575,195,605" href="backend_dirs.html#utils" alt="utils" />
<area coords="235,575,360,605" href="backend_dirs.html#catalog" alt="catalog" />
<area coords="405,575,525,605" href="backend_dirs.html#storage" alt="storage" />
<area coords="155,635,275,665" href="backend_dirs.html#access" alt="access" />
<area coords="325,635,450,665" href="backend_dirs.html#nodes" alt="nodes" />
<area coords="75,705,200,730" href="backend_dirs.html#bootstrap" alt="bootstrap" />
</map>
<br />
<p>A query comes to the backend via data packets arriving through
TCP/IP or Unix Domain sockets. It is loaded into a string, and
passed to the <a href="../../backend/parser">parser,</a> where the
lexical scanner, <a href="../../backend/parser/scan.l">scan.l,</a>
breaks the query up into tokens(words). The parser uses <a
href="../../backend/parser/gram.y">gram.y</a> and the tokens to
identify the query type, and load the proper query-specific
structure, like <a
href="../../include/nodes/parsenodes.h">CreateStmt</a> or <a
href="../../include/nodes/parsenodes.h">SelectStmt.</a></p>
<p>The statement is then identified as complex (<i>SELECT / INSERT /
UPDATE / DELETE</i>) or a simple, e.g <i> CREATE USER, ANALYZE, </i>,
etc. Utility commands are processed by statement-specific functions in <a
href="../../backend/commands">backend/commands.</a> Complex statements
require more handling.</p>
<p>The parser takes a complex query, and creates a <a
href="../../include/nodes/parsenodes.h">Query</a> structure that
contains all the elements used by complex queries. Query.qual holds
the <i>WHERE</i> clause qualification, which is filled in by <a
href="../../backend/parser/parse_clause.c">transformWhereClause().</a>
Each table referenced in the query is represented by a <a
href="../../include/nodes/parsenodes.h">RangeTableEntry,</a> and
they are linked together to form the <i>range table</i> of the
query, which is generated by <a
href="../../backend/parser/parse_clause.c">transformFromClause().</a>
Query.rtable holds the query's range table.</p>
<p>Certain queries, like <i>SELECT,</i> return columns of data.
Other queries, like <i>INSERT</i> and <i>UPDATE,</i> specify the
columns modified by the query. These column references are
converted to <a
href="../../include/nodes/primnodes.h">TargetEntry</a> entries,
which are linked together to make up the <i>target list</i> of the
query. The target list is stored in Query.targetList, which is
generated by <a
href="../../backend/parser/parse_target.c">transformTargetList().</a></p>
<p>Other query elements, like aggregates(<i>SUM()</i>), <i>GROUP
BY,</i> and <i>ORDER BY</i> are also stored in their own Query
fields.</p>
<p>The next step is for the Query to be modified by any
<i>VIEWS</i> or <i>RULES</i> that may apply to the query. This is
performed by the <a href="../../backend/rewrite">rewrite</a>
system.</p>
<p>The <a href="../../backend/optimizer">optimizer</a> takes the
Query structure and generates an optimal <a
href="../../include/nodes/plannodes.h">Plan,</a> which contains the
operations to be performed to execute the query. The <a
href="../../backend/optimizer/path">path</a> module determines the
best table join order and join type of each table in the
RangeTable, using Query.qual(<i>WHERE</i> clause) to consider
optimal index usage.</p>
<p>The Plan is then passed to the <a
href="../../backend/executor">executor</a> for execution, and the
result returned to the client. The Plan actually as set of nodes,
arranged in a tree structure with a top-level node, and various
sub-nodes as children.</p>
<p>There are many other modules that support this basic
functionality. They can be accessed by clicking on the
flowchart.</p>
<hr />
<p>Another area of interest is the shared memory area, which
contains data accessable to all backends. It has recently used
data/index blocks, locks, backend process information, and lookup
tables for these structures:</p>
<ul>
<li>ShmemIndex - lookup shared memory addresses using structure
names</li>
<li><a href="../../include/storage/buf_internals.h">Buffer
Descriptor</a> - control header for buffer cache block</li>
<li><a href="../../include/storage/buf_internals.h">Buffer
Block</a> - data/index buffer cache block</li>
<li>Shared Buffer Lookup Table - lookup of buffer cache block
addresses using table name and block number( <a
href="../../include/storage/buf_internals.h">BufferTag</a>)</li>
<li>MultiLevelLockTable (ctl) - control structure for each locking
method. Currently, only multi-level locking is used(<a
href="../../include/storage/lock.h">LOCKMETHODCTL</a>).</li>
<li>MultiLevelLockTable (lock hash) - the <a
href="../../include/storage/lock.h">LOCK</a> structure, looked up
using relation, database object ids(<a
href="../../include/storage/lock.h">LOCKTAG)</a>. The lock table
structure contains the lock modes(read/write or shared/exclusive)
and circular linked list of backends (<a
href="../../include/storage/proc.h">PROC</a> structure pointers)
waiting on the lock.</li>
<li>MultiLevelLockTable (xid hash) - lookup of LOCK structure
address using transaction id, LOCK address. It is used to quickly
check if the current transaction already has any locks on a table,
rather than having to search through all the held locks. It also
stores the modes (read/write) of the locks held by the current
transaction. The returned <a
href="../../include/storage/lock.h">XIDLookupEnt</a> structure also
contains a pointer to the backend's PROC.lockQueue.</li>
<li><a href="../../include/storage/proc.h">Proc Header</a> -
information about each backend, including locks held/waiting,
indexed by process id</li>
</ul>
<p>Each data structure is created by calling <a
href="../../backend/storage/ipc/shmem.c">ShmemInitStruct(),</a> and
the lookups are created by <a
href="../../backend/storage/ipc/shmem.c">ShmemInitHash().</a></p>
<hr />
<small>Maintainer: Bruce Momjian (<a
href="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</a>)<br />
Last updated: Fri May 6 14:22:27 EDT 2005</small>
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