<HTML>
<HEAD>
-<TITLE>PostgreSQL Backend Flowchart</TITLE>
+<TITLE>How PostgreSQL Processes a Query</TITLE>
</HEAD>
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#A00000" ALINK="#0000FF">
<H1 ALIGN=CENTER>
-PostgreSQL Backend Flowchart
+How PostgreSQL Processes a Query
</H1>
<H2 ALIGN=CENTER>
by Bruce Momjian
</H2>
<P>
-A query come into the backend via data packets coming in through TCP/IP
-and Unix Domain sockets. It is loaded into a string, and passed to
+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 or SelectStmt.</A>
+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>
-The query is then identified as a <I>Utility</I> function 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 <CODE>SELECT, UPDATE,</CODE> and
+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
+<CODE>SELECT, UPDATE,</CODE> and
<CODE>DELETE</CODE> requires much more handling.
<P>
The parser takes a complex query, and creates 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">
-makeRangeTable().</A> Query.rtable holds the queries range table.
+makeRangeTable().</A> Query.rtable holds the query's range table.
<P>
Certain queries, like <CODE>SELECT,</CODE> return columns of data. Other
queries, like <CODE>INSERT</CODE> and <CODE>UPDATE,</CODE> specify the columns
<A HREF="../../backend/parser/parse_target.c">transformTargetList().</A>
<P>
Other query elements, like aggregates(<CODE>SUM()</CODE>), <CODE>GROUP BY,</CODE>
-<CODE>ORDER BY</CODE> are also stored in their own Query fields.
+and <CODE>ORDER BY</CODE> are also stored in their own Query fields.
<P>
The next step is for the Query to be modified by any <CODE>VIEWS</CODE> or
<CODE>RULES</CODE> that may apply to the query. This is performed by the <A
There are many other modules that support this basic functionality.
They can be accessed by clicking on the flowchart.
<P>
-Another area of interest is the shared memory area, containing table
-data/index blocks, locks, backend information, and lookup tables for
-these structures:
+Another area of interest is the shared memory area, which contains data
+accessable to all backends. It has table recently used data/index
+blocks, locks, backend information, and lookup tables for these
+structures:
<UL>
<LI>ShmemIndex - lookup shared memory addresses using structure names
<LI><A HREF="../../include/storage/buf_internals.h">Buffer
-Descriptors</A> - control header for buffer cache block
-<LI><A HREF="../../include/storage/buf_internals.h">Buffer Blocks</A> -
+Descriptor</A> - control header for buffer cache block
+<LI><A HREF="../../include/storage/buf_internals.h">Buffer Block</A> -
data/index buffer cache block
-<LI>Shared Buf Lookup Table - lookup of buffer cache block address using
+<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>MultiLevelLockTable (ctl) - <A
-HREF="../../include/storage/lock.h">LOCKCTL</A> control structure for
-each locking method. Currently, only multi-level locking is used.
+<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>MultiLevelLockTable (lock hash) - the <A
HREF="../../include/storage/lock.h">LOCK</A> structure, looked up using
relation, database object ids(<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>
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+<P>
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