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<chapter id="overview">

<para>

Figure \ref{parsetree} shows the <firstterm>parse tree</firstterm> built by the

grammar rules and actions given in <filename>gram.y</filename> for the query

given in <xref linkend="simple-select" endterm="simple-select">

given in <xref linkend="simple-select">

(without the <firstterm>operator tree</firstterm> for

the <firstterm>where clause</firstterm> which is shown in figure \ref{where_clause}

because there was not enough space to show both data structures in one

<para>

Figure \ref{where_clause} shows the operator tree built for the

where clause of the SQL query given in example

<xref linkend="simple-select" endterm="simple-select">

where clause of the SQL query given in

<xref linkend="simple-select">

which is attached to the field

<literal>qual</literal> of the <literal>SelectStmt</literal> node. The top node of the

operator tree is an <literal>A_Expr</literal> node representing an <literal>AND</literal>

within a cluster make calls to the library.

The library sends user requests over the network to the

<Application>postmaster</Application>

(<XRef LinkEnd="PGARCH-CONNECTIONS" EndTerm="PGARCH-CONNECTIONS">(a)),

(<XRef LinkEnd="PGARCH-CONNECTIONS">(a)),

which in turn starts a new backend server process

(<XRef LinkEnd="PGARCH-CONNECTIONS" EndTerm="PGARCH-CONNECTIONS">(b))

(<XRef LinkEnd="PGARCH-CONNECTIONS">(b))

<Figure Id="PGARCH-CONNECTIONS">

<Title>How a connection is established</Title>

<Graphic Align="center" FileRef="connections.gif" Format="GIF"></Graphic>

</Figure>

and connects the frontend process to the new server

(<XRef LinkEnd="PGARCH-CONNECTIONS" EndTerm="PGARCH-CONNECTIONS">(c)).

(<XRef LinkEnd="PGARCH-CONNECTIONS">(c)).

From that point on, the frontend process and the backend

server communicate without intervention by the

<Application>postmaster</Application>. Hence, the <Application>postmaster</Application> is always running, waiting

applications that wish to access a given database

within a cluster make calls to the library.

The library sends user requests over the network to the

<Application>postmaster</Application> (<XRef LinkEnd="ARCH-CLIENTSERVER" EndTerm="ARCH-CLIENTSERVER">),

<Application>postmaster</Application> (<XRef LinkEnd="ARCH-CLIENTSERVER">),

which in turn starts a new backend server process

<Figure Id="ARCH-CLIENTSERVER">

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<chapter id="extend">

The Reference Manual gives a more detailed explanation

of these catalogs and their attributes. However,

<xrefendterm="EXTEND-CATALOGS"linkend="EXTEND-CATALOGS">

<xref linkend="EXTEND-CATALOGS">

shows the major entities and their relationships

in the system catalogs. (Attributes that do not refer

to other entities are not shown unless they are part of

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<chapter id="libpqplusplus">

<title>libpq - C++ Binding Library</title>

<title>libpq++ - C++ Binding Library</title>

<para>

<filename>libpq++</filename> is the C++ API to

an application program:

<note>

<para>

Refer tothe<xref endterm="libpq" linkend="libpq-envars"> for a complete

Refer to <xref linkend="libpq-envars"> for a complete

list of available connection options.

</para>

</note>

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<chapter id="xfunc">

There are currently three procedural languages available in the standard

<productname>Postgres</productname> distribution (PLSQL, PLTCL and

PLPERL), and other languages can be defined.

Refer to <xref linkend="xplang-title" endterm="xplang-title"> for

Refer to <xref linkend="xplang"> for

more information.

</para>

</sect1>

it can be dynamically loaded into

<productname>Postgres</productname>

always requires special flags.

See <xref linkend="dfunc-title" endterm="dfunc-title">

See <xref linkend="dfunc">

for a detailed explanation of how to do it for

your particular operating system.

</para>

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Postgres documentation

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<para>

Look back at

<xrefendterm="EXTEND-CATALOGS"linkend="EXTEND-CATALOGS">.

<xref linkend="EXTEND-CATALOGS">.

The right half shows the catalogs that we must modify in order to tell

<productname>Postgres</productname> how to use a user-defined type and/or

user-defined operators with an index (i.e., <filename>pg_am, pg_amop,