the

Unix Domain sockets. It is loaded into a string, and passed tothe

<AHREF="../../backend/parser">parser,</A> where the lexical scanner,

<AHREF="../../backend/parser/scan.l">scan.l,</A>

breaks the query up into tokens(words). The parser

uses

<AHREF="../../backend/parser/gram.y">gram.y</A> and the tokens to

identify the query type, and load the proper query-specific structure,

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

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

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

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

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

<I>SELECT, UPDATE,</I> and<I>DELETE</I> requires much more handling.<P>

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

<I>WHERE</I> clause qualification, which is filled in by

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

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

are linked together to form the<I>range table</I> of the query, which

isgenerated by<AHREF="../../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<AHREF="../../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

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

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.

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.

HREF="../../backend/rewrite">rewrite</A> system.<P>

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

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.

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.

children.<P>

There are many other modules that support this basic functionality.

They can be accessed by clicking on the flowchart.

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, which contains data

accessable to all backends. It has table recently used data/index

blocks, locks, backend information, and lookup tables for these

structures:

<LI>ShmemIndex - lookup shared memory addresses using structure names

<LI><AHREF="../../include/storage/buf_internals.h">Buffer

Descriptor</A> - control header for buffer cache block

<LI><AHREF="../../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(<AHREF="../../include/storage/buf_internals.h">

BufferTag</A>)

<LI>MultiLevelLockTable (ctl) - control structure for

each lockingmethod. Currently, only multi-level locking is used(<A

<LI>Shared Buffer Lookup Table - lookup of buffer cache block addresses

usingtable 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><AHREF="../../include/storage/proc.h">Proc Header</A> - information

about each backend, including locks held/waiting, indexed by process id

Each data structure is created by calling<A

HREF="../../backend/storage/ipc/shmem.c">ShmemInitStruct(),</A> and

thelookups are created by

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