by Bruce Momjian

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

like<AHREF="../../include/nodes/parsenodes.h">CreateStmt</A> or<A

HREF="../../include/nodes/parsenodes.h">SelectStmt.</A>

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<AHREF="../../backend/commands"> commands.</A> A complex query, like

<I>SELECT, UPDATE,</I> and

<I>DELETE</I> requires much more handling.

The parser takes a complex query, and creates a

<AHREF="../../include/nodes/parsenodes.h">Query</A> structure that

contains all the elements used by complex queries. Query.qual holds the

generated by<AHREF="../../backend/parser/parse_clause.c">

makeRangeTable().</A> Query.rtable holds the query's range table.

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

generated by

<AHREF="../../backend/parser/parse_target.c">transformTargetList().</A>

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.

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.

The<AHREF="../../backend/optimizer">optimizer</A> takes the Query

structure and generates an optimal<A

HREF="../..//include/nodes/plannodes.h">Plan,</A> which contains the

Query.qual(<I>WHERE</I> clause) to consider optimal index usage.

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.

There are many other modules that support this basic functionality.

They can be accessed by clicking on the flowchart.

Click on an item to see more detail or look at the full

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

the lookups are created by

<AHREF="../../backend/storage/ipc/shmem.c">ShmemInitHash().</A>

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