<HR>
<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 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>
+<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>
+
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>
+<I>SELECT, UPDATE,</I> and <I>DELETE</I> requires much more handling.<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>
+<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">
-makeRangeTable().</A> Query.rtable holds the query's range table.
-<P>
+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 query's range table.<P>
+
Certain queries, like <I>SELECT,</I> return columns of data. Other
queries, like <I>INSERT</I> and <I>UPDATE,</I> specify the columns
placed in <A HREF="../../include/nodes/parsenodes.h">target list
entries,</I> and 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>
+generated by <A
+HREF="../../backend/parser/parse_target.c">transformTargetList().</A><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>
+and <I>ORDER BY</I> are also stored in their own Query fields.<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>
+HREF="../../backend/rewrite">rewrite</A> system.<P>
+
The <A HREF="../../backend/optimizer">optimizer</A> takes the Query
structure and generates an optimal <A
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>
+Query.qual(<I>WHERE</I> clause) to consider optimal index usage.<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>
+children.<P>
-There are many other modules that support this basic functionality.
-They can be accessed by clicking on the flowchart.
-<P>
-<HR>
-<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 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
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 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) - control structure for
-
each locking method. Currently, only multi-level locking is used(<A
+<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) - 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="../../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/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>MultiLevelLockTable (xid hash) - lookup of LOCK structure address
<LI><A HREF="../../include/storage/proc.h">Proc Header</A> - information
about each backend, including locks held/waiting, indexed by process id
</UL>
+
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>
+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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