<listitem>

<para>

Allows for mapping between system and database user names. See

<xref linkend="auth-username-maps"/> for details. Fora SSPI/Kerberos

<xref linkend="auth-username-maps"/> for details. Foran SSPI/Kerberos

principal, such as <literal>username@EXAMPLE.COM</literal> (or, less

commonly, <literal>username/hostbased@EXAMPLE.COM</literal>), the

user name used for mapping is

In general, enclose the value in single quotes, doubling any single

quotes within the value. Quotes can usually be omitted if the value

is a simple number or identifier, however.

(Values that matcha SQL keyword require quoting in some contexts.)

(Values that matchan SQL keyword require quoting in some contexts.)

</para>

</listitem>

<productname>PostgreSQL</productname> provides three SQL

commands to establish configuration defaults.

The already-mentioned <command>ALTER SYSTEM</command> command

providesa SQL-accessible means of changing global defaults; it is

providesan SQL-accessible means of changing global defaults; it is

functionally equivalent to editing <filename>postgresql.conf</filename>.

In addition, there are two commands that allow setting of defaults

on a per-database or per-role basis:

<literal>\'</literal>. However, use of <literal>\'</literal> creates security risks

because in some client character set encodings, there are multibyte

characters in which the last byte is numerically equivalent to ASCII

<literal>\</literal>. If client-side code does escaping incorrectly thena

<literal>\</literal>. If client-side code does escaping incorrectly thenan

SQL-injection attack is possible. This risk can be prevented by

making the server reject queries in which a quote mark appears to be

escaped by a backslash.

<para>

The reason that single quotes must be doubled, as shown

in <xref linkend="datatype-binary-sqlesc"/>, is that this

is true for any string literal ina SQL command. The generic

is true for any string literal inan SQL command. The generic

string-literal parser consumes the outermost single quotes

and reduces any pair of single quotes to one data character.

What the <type>bytea</type> input function sees is just one

<title>Return Value</title>

<para>

For an async query (that is,a SQL statement returning rows),

For an async query (that is,an SQL statement returning rows),

the function returns the row(s) produced by the query. To use this

function, you will need to specify the expected set of columns,

as previously discussed for <function>dblink</function>.

</para>

<para>

For an async command (that is,a SQL statement not returning rows),

For an async command (that is,an SQL statement not returning rows),

the function returns a single row with a single text column containing

the command's status string. It is still necessary to specify that

the result will have a single text column in the calling <literal>FROM</literal>

<para>

<function>dblink_build_sql_insert</function> can be useful in doing selective

replication of a local table to a remote database. It selects a row

from the local table based on primary key, and then buildsa SQL

from the local table based on primary key, and then buildsan SQL

<command>INSERT</command> command that will duplicate that row, but with

the primary key values replaced by the values in the last argument.

(To make an exact copy of the row, just specify the same values for

<para>

<function>dblink_build_sql_delete</function> can be useful in doing selective

replication of a local table to a remote database. It buildsa SQL

replication of a local table to a remote database. It buildsan SQL

<command>DELETE</command> command that will delete the row with the given

primary key values.

</para>

<para>

<function>dblink_build_sql_update</function> can be useful in doing selective

replication of a local table to a remote database. It selects a row

from the local table based on primary key, and then buildsa SQL

from the local table based on primary key, and then buildsan SQL

<command>UPDATE</command> command that will duplicate that row, but with

the primary key values replaced by the values in the last argument.

(To make an exact copy of the row, just specify the same values for

</para>

<para>

The third option is to declarea SQL identifier linked to

The third option is to declarean SQL identifier linked to

the connection, for example:

<programlisting>

EXEC SQL AT <replaceable>connection-name</replaceable> DECLARE <replaceable>statement-name</replaceable> STATEMENT;

EXEC SQL PREPARE <replaceable>statement-name</replaceable> FROM :<replaceable>dyn-string</replaceable>;

</programlisting>

Once you linka SQL identifier to a connection, you execute dynamic SQL

Once you linkan SQL identifier to a connection, you execute dynamic SQL

without an AT clause. Note that this option behaves like preprocessor

directives, therefore the link is enabled only in the file.

</para>

variables and vice-versa. However, when creating a statement ecpg does

not know the types of the columns, so that it cannot check if a C array

is input into a corresponding SQL-level array. When processing the

output ofa SQL statement, ecpg has the necessary information and thus

output ofan SQL statement, ecpg has the necessary information and thus

checks if both are arrays.

</para>

parameters to a prepared query are:

</para>

<step><simpara>Create a prepared query (prepared statement)</simpara></step>

<step><simpara>Declarea sqlda_t structure as an input SQLDA.</simpara></step>

<step><simpara>Declarean sqlda_t structure as an input SQLDA.</simpara></step>

<step><simpara>Allocate memory area (as sqlda_t structure) for the input SQLDA.</simpara></step>

<step><simpara>Set (copy) input values in the allocated memory.</simpara></step>

<step><simpara>Open a cursor with specifying the input SQLDA.</simpara></step>

EXEC SQL PREPARE stmt1 FROM :query;

EXEC SQL DECLARE cur1 CURSOR FOR stmt1;

/* Createa SQLDA structure for an input parameter */

/* Createan SQLDA structure for an input parameter */

sqlda2 = (sqlda_t *)malloc(sizeof(sqlda_t) + sizeof(sqlvar_t));

memset(sqlda2, 0, sizeof(sqlda_t) + sizeof(sqlvar_t));

sqlda2->sqln = 2; /* a number of input variables */

<title>Description</title>

<para>

<command>DECLARE STATEMENT</command> declaresa SQL statement identifier.

<command>DECLARE STATEMENT</command> declaresan SQL statement identifier.

SQL statement identifier can be associated with the connection.

When the identifier is used by dynamic SQL statements, the statements

are executed using the associated connection.

<term><replaceable class="parameter">statement_name</replaceable></term>

<listitem>

<para>

The name ofa SQL statement identifier, either as an SQL identifier or a host variable.

The name ofan SQL statement identifier, either as an SQL identifier or a host variable.

</para>

</listitem>

</varlistentry>

and include the global <acronym>PGXS</acronym> makefile.

Here is an example that builds an extension module named

<literal>isbn_issn</literal>, consisting of a shared library containing

some C code, an extension control file,a SQL script, an include file

some C code, an extension control file,an SQL script, an include file

(only needed if other modules might need to access the extension functions

without going via SQL), and a documentation text file:

<programlisting>

<returnvalue>json</returnvalue>

</para>

<para>

Convertsa SQL array to a JSON array. The behavior is the same

Convertsan SQL array to a JSON array. The behavior is the same

as <function>to_json</function> except that line feeds will be added

between top-level array elements if the optional boolean parameter is

true.

<returnvalue>json</returnvalue>

</para>

<para>

Convertsa SQL composite value to a JSON object. The behavior is the

Convertsan SQL composite value to a JSON object. The behavior is the

same as <function>to_json</function> except that line feeds will be

added between top-level elements if the optional boolean parameter is

true.

<itemizedlist spacing="compact">

<listitem>

<para>

A JSON null value is converted toa SQL null in all cases.

A JSON null value is converted toan SQL null in all cases.

</para>

</listitem>

<listitem>

<literal>string</literal>, <literal>number</literal>,

<literal>boolean</literal>, and <literal>null</literal>.

(The <literal>null</literal> result should not be confused

witha SQL NULL; see the examples.)

withan SQL NULL; see the examples.)

</para>

<para>

<literal>json_typeof('-123.4')</literal>

Currently, temporary table creation is not allowed during read only

transactions, so in some cases existing scripts will not run correctly.

This restriction might be relaxed in a later release. This is

botha SQL Standard compliance issue and a technical issue.

bothan SQL Standard compliance issue and a technical issue.

</para>

<para>

<para>

If the index AM wishes to cache data across successive index insertions

withina SQL statement, it can allocate space

withinan SQL statement, it can allocate space

in <literal>indexInfo-&gt;ii_Context</literal> and store a pointer to the

data in <literal>indexInfo-&gt;ii_AmCache</literal> (which will be NULL

initially).

<listitem>

<para>

The text of a failed internally-generated command. This could

be, for example,a SQL query issued by a PL/pgSQL function.

be, for example,an SQL query issued by a PL/pgSQL function.

</para>

</listitem>

</varlistentry>

</indexterm>

<para>

Ordinarily, <application>libpq</application> collectsa SQL command's

Ordinarily, <application>libpq</application> collectsan SQL command's

entire result and returns it to the application as a single

<structname>PGresult</structname>. This can be unworkable for commands

that return a large number of rows. For such cases, applications can use