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Update backend flowchard wording
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Bruce Momjian
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<HTML>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<TITLE>How PostgreSQL Processes a Query</TITLE>
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<html xmlns="http://www.w3.org/1999/xhtml">
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<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#A00000" ALINK="#0000FF">
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<meta name="generator"
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<H1>
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content="HTML Tidy for BSD/OS (vers 1st July 2002), see www.w3.org" />
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How PostgreSQL Processes a Query
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<title>How PostgreSQL Processes a Query</title>
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</H1>
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<H2>
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<body bgcolor="#FFFFFF" text="#000000" link="#FF0000"
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by Bruce Momjian
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vlink="#A00000" alink="#0000FF">
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</H2>
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<h1>How PostgreSQL Processes a Query</h1>
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<P>
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<AREA coords="75,575,195,605" HREF="backend_dirs.html#utils" alt="utils"></AREA>
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<AREA coords="75,705,200,730" HREF="backend_dirs.html#bootstrap" alt="bootstrap"></AREA>
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</MAP>
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<EM>
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Click on an item to see more detail or look at the full
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<A HREF="backend_dirs.html">index.</A>
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</EM>
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<BR>
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<BR>
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</P>
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<P>
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A query comes to the backend via data packets arriving through TCP/IP or
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<h2>by Bruce Momjian</h2>
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Unix Domain sockets. It is loaded into a string, and passed to the
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<A HREF="../../backend/parser">parser,</A> where the lexical scanner,
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<A HREF="../../backend/parser/scan.l">scan.l,</A> breaks the query up
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into tokens(words). The parser uses <A
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HREF="../../backend/parser/gram.y">gram.y</A> and the tokens to identify
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the query type, and load the proper query-specific structure, like <A
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HREF="../../include/nodes/parsenodes.h">CreateStmt</A> or <A
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HREF="../../include/nodes/parsenodes.h">SelectStmt.</A></P><P>
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<p><img src="flow.gif" usemap="#flowmap" alt="flowchart" />
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The query is then identified as a <I>Utility</I> query or a more complex
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<em>Click on an item to see more detail or look at the full
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query. A <I>Utility</I> query is processed by a query-specific function
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<a href="backend_dirs.html">index.</a></em>
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in <A HREF="../../backend/commands"> commands.</A> A complex query, like
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<I>SELECT, UPDATE,</I> and <I>DELETE</I> requires much more handling.</P><P>
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<map name="flowmap" id="flowmap">
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<area coords="125,35,245,65" href="backend_dirs.html#main" alt="main" />
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<area coords="125,100,245,125" href="backend_dirs.html#postmaster" alt="postmaster" />
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<area coords="325,65,450,95" href="backend_dirs.html#libpq" alt="libpq" />
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<area coords="125,160,245,190" href="backend_dirs.html#tcop" alt="tcop" />
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<area coords="325,160,450,190" href="backend_dirs.html#tcop" alt="tcop" />
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<area coords="125,240,245,265" href="backend_dirs.html#parser" alt="parser" />
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<area coords="125,300,250,330" href="backend_dirs.html#tcop" alt="tcop" />
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<area coords="125,360,250,390" href="backend_dirs.html#optimizer" alt="optimizer" />
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<area coords="125,425,245,455" href="backend_dirs.html#optimizer_plan" alt="plan" />
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<area coords="125,490,245,515" href="backend_dirs.html#executor" alt="executor" />
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<area coords="325,300,450,330" href="backend_dirs.html#commands" alt="commands" />
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<area coords="75,575,195,605" href="backend_dirs.html#utils" alt="utils" />
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<area coords="235,575,360,605" href="backend_dirs.html#catalog" alt="catalog" />
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<area coords="405,575,525,605" href="backend_dirs.html#storage" alt="storage" />
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<area coords="155,635,275,665" href="backend_dirs.html#access" alt="access" />
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<area coords="325,635,450,665" href="backend_dirs.html#nodes" alt="nodes" />
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<area coords="75,705,200,730" href="backend_dirs.html#bootstrap" alt="bootstrap" />
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</map>
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The parser takes a complex query, and creates a
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<br />
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<A HREF="../../include/nodes/parsenodes.h">Query</A> structure that
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contains all the elements used by complex queries. Query.qual holds the
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<I>WHERE</I> clause qualification, which is filled in by <A
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HREF="../../backend/parser/parse_clause.c">transformWhereClause().</A>
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Each table referenced in the query is represented by a <A
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HREF="../../include/nodes/parsenodes.h"> RangeTableEntry,</A> and they
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are linked together to form the <I>range table</I> of the query, which
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is generated by <A HREF="../../backend/parser/parse_clause.c">
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transformFromClause().</A> Query.rtable holds the query's range table.</P><P>
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<p>A query comes to the backend via data packets arriving through
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TCP/IP or Unix Domain sockets. It is loaded into a string, and
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passed to the <a href="../../backend/parser">parser,</a> where the
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lexical scanner, <a href="../../backend/parser/scan.l">scan.l,</a>
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breaks the query up into tokens(words). The parser uses <a
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href="../../backend/parser/gram.y">gram.y</a> and the tokens to
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identify the query type, and load the proper query-specific
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structure, like <a
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href="../../include/nodes/parsenodes.h">CreateStmt</a> or <a
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href="../../include/nodes/parsenodes.h">SelectStmt.</a></p>
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Certain queries, like <I>SELECT,</I> return columns of data. Other
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<p>The statement is then identified as complex (<i>SELECT / INSERT /
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queries, like <I>INSERT</I> and <I>UPDATE,</I> specify the columns
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UPDATE / DELETE</i>) or a simple, e.g <i> CREATE USER, ANALYZE, </i>,
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modified by the query. These column references are converted to <A
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etc. Utility commands are processed by statement-specific functions in <a
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HREF="../../include/nodes/primnodes.h">TargetEntry</A> entries, which are
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href="../../backend/commands">backend/commands.</a> Complex statements
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linked together to make up the <I>target list</I> of
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require more handling.</p>
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the query. The target list is stored in Query.targetList, which is
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generated by <A
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HREF="../../backend/parser/parse_target.c">transformTargetList().</A></P><P>
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<p>The parser takes a complex query, and creates a <a
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href="../../include/nodes/parsenodes.h">Query</a> structure that
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contains all the elements used by complex queries. Query.qual holds
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the <i>WHERE</i> clause qualification, which is filled in by <a
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href="../../backend/parser/parse_clause.c">transformWhereClause().</a>
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Each table referenced in the query is represented by a <a
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href="../../include/nodes/parsenodes.h">RangeTableEntry,</a> and
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they are linked together to form the <i>range table</i> of the
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query, which is generated by <a
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href="../../backend/parser/parse_clause.c">transformFromClause().</a>
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Query.rtable holds the query's range table.</p>
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Other query elements, like aggregates(<I>SUM()</I>), <I>GROUP BY,</I>
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<p>Certain queries, like <i>SELECT,</i> return columns of data.
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and <I>ORDER BY</I> are also stored in their own Query fields.</P><P>
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Other queries, like <i>INSERT</i> and <i>UPDATE,</i> specify the
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columns modified by the query. These column references are
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converted to <a
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href="../../include/nodes/primnodes.h">TargetEntry</a> entries,
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which are linked together to make up the <i>target list</i> of the
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query. The target list is stored in Query.targetList, which is
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generated by <a
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href="../../backend/parser/parse_target.c">transformTargetList().</a></p>
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<p>Other query elements, like aggregates(<i>SUM()</i>), <i>GROUP
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BY,</i> and <i>ORDER BY</i> are also stored in their own Query
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fields.</p>
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The next step is for the Query to be modified by any <I>VIEWS</I> or
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<p>The next step is for the Query to be modified by any
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<I>RULES</I> that may apply to the query. This is performed by the <A
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<i>VIEWS</i> or <i>RULES</i> that may apply to the query. This is
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HREF="../../backend/rewrite">rewrite</A> system.</P><P>
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performed by the <a href="../../backend/rewrite">rewrite</a>
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system.</p>
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<p>The <a href="../../backend/optimizer">optimizer</a> takes the
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Query structure and generates an optimal <a
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href="../../include/nodes/plannodes.h">Plan,</a> which contains the
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operations to be performed to execute the query. The <a
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href="../../backend/optimizer/path">path</a> module determines the
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best table join order and join type of each table in the
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RangeTable, using Query.qual(<i>WHERE</i> clause) to consider
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optimal index usage.</p>
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The <A HREF="../../backend/optimizer">optimizer</A> takes the Query
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<p>The Plan is then passed to the <a
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structure and generates an optimal <A
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href="../../backend/executor">executor</a> for execution, and the
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HREF="../../include/nodes/plannodes.h">Plan,</A> which contains the
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result returned to the client. The Plan actually as set of nodes,
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operations to be performed to execute the query. The <A
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arranged in a tree structure with a top-level node, and various
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HREF="../../backend/optimizer/path">path</A> module determines the best
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sub-nodes as children.</p>
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table join order and join type of each table in the RangeTable, using
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Query.qual(<I>WHERE</I> clause) to consider optimal index usage.</P><P>
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<p>There are many other modules that support this basic
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functionality. They can be accessed by clicking on the
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flowchart.</p>
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The Plan is then passed to the <A
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<hr />
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HREF="../../backend/executor">executor</A> for execution, and the result
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<p>Another area of interest is the shared memory area, which
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returned to the client. The Plan actually as set of nodes, arranged in
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contains data accessable to all backends. It has recently used
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a tree structure with a top-level node, and various sub-nodes as
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data/index blocks, locks, backend process information, and lookup
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children.</P><P>
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tables for these structures:</p>
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There are many other modules that support this basic functionality. They
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<ul>
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can be accessed by clicking on the flowchart.</P>
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<li>ShmemIndex - lookup shared memory addresses using structure
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names</li>
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<li><a href="../../include/storage/buf_internals.h">Buffer
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Descriptor</a> - control header for buffer cache block</li>
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<HR><P>
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<li><a href="../../include/storage/buf_internals.h">Buffer
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Block</a> - data/index buffer cache block</li>
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<li>Shared Buffer Lookup Table - lookup of buffer cache block
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addresses using table name and block number( <a
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href="../../include/storage/buf_internals.h">BufferTag</a>)</li>
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Another area of interest is the shared memory area, which contains data
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<li>MultiLevelLockTable (ctl) - control structure for each locking
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accessable to all backends. It has recently used data/index blocks,
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method. Currently, only multi-level locking is used(<a
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locks, backend process information, and lookup tables for these
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href="../../include/storage/lock.h">LOCKMETHODCTL</a>).</li>
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structures:
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</P>
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<UL>
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<li>MultiLevelLockTable (lock hash) - the <a
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<LI>ShmemIndex - lookup shared memory addresses using structure names</LI>
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href="../../include/storage/lock.h">LOCK</a> structure, looked up
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<LI><A HREF="../../include/storage/buf_internals.h">Buffer
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using relation, database object ids(<a
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Descriptor</A> - control header for buffer cache block</LI>
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href="../../include/storage/lock.h">LOCKTAG)</a>. The lock table
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<LI><A HREF="../../include/storage/buf_internals.h">Buffer Block</A> -
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structure contains the lock modes(read/write or shared/exclusive)
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data/index buffer cache block</LI>
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and circular linked list of backends (<a
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<LI>Shared Buffer Lookup Table - lookup of buffer cache block addresses
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href="../../include/storage/proc.h">PROC</a> structure pointers)
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using table name and block number(<A
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waiting on the lock.</li>
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HREF="../../include/storage/buf_internals.h"> BufferTag</A>)</LI>
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<LI>MultiLevelLockTable (ctl) - control structure for each locking
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method. Currently, only multi-level locking is used(<A
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HREF="../../include/storage/lock.h">LOCKMETHODCTL</A>).</LI>
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<LI>MultiLevelLockTable (lock hash) - the <A
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HREF="../../include/storage/lock.h">LOCK</A> structure, looked up using
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relation, database object ids(<A
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HREF="../../include/storage/lock.h">LOCKTAG)</A>. The lock table
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structure contains the lock modes(read/write or shared/exclusive) and
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circular linked list of backends (<A
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HREF="../../include/storage/proc.h">PROC</A> structure pointers) waiting
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on the lock.</LI>
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<LI>MultiLevelLockTable (xid hash) - lookup of LOCK structure address
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using transaction id, LOCK address. It is used to quickly check if the
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current transaction already has any locks on a table, rather than having
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to search through all the held locks. It also stores the modes
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(read/write) of the locks held by the current transaction. The returned
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<A HREF="../../include/storage/lock.h">XIDLookupEnt</A> structure also
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contains a pointer to the backend's PROC.lockQueue.</LI>
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<LI><A HREF="../../include/storage/proc.h">Proc Header</A> - information
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about each backend, including locks held/waiting, indexed by process id</LI>
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</UL>
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<P>Each data structure is created by calling <A
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<li>MultiLevelLockTable (xid hash) - lookup of LOCK structure
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HREF="../../backend/storage/ipc/shmem.c">ShmemInitStruct(),</A> and the
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address using transaction id, LOCK address. It is used to quickly
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lookups are created by <A
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check if the current transaction already has any locks on a table,
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HREF="../../backend/storage/ipc/shmem.c">ShmemInitHash().</A></P>
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rather than having to search through all the held locks. It also
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stores the modes (read/write) of the locks held by the current
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transaction. The returned <a
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href="../../include/storage/lock.h">XIDLookupEnt</a> structure also
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contains a pointer to the backend's PROC.lockQueue.</li>
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<li><a href="../../include/storage/proc.h">Proc Header</a> -
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information about each backend, including locks held/waiting,
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indexed by process id</li>
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</ul>
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<HR>
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<p>Each data structure is created by calling <a
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<SMALL>
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href="../../backend/storage/ipc/shmem.c">ShmemInitStruct(),</a> and
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Maintainer: Bruce Momjian (<A
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the lookups are created by <a
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HREF="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
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href="../../backend/storage/ipc/shmem.c">ShmemInitHash().</a></p>
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Last updated: Mon Aug 10 10:48:06 EDT 1998
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</SMALL>
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<hr />
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</BODY>
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<small>Maintainer: Bruce Momjian (<a
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</HTML>
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href="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</a>)<br />
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Last updated: Fri May 6 14:22:27 EDT 2005</small>
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</body>
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</html>
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