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Storage for session variables and SQL interface
Session variables are stored in session memory in a dedicated hash table. They are set by the LET command and read by the SELECT command. The access rights should be checked. The identifiers of session variables should always be shadowed by possible column identifiers: we don't want to break an application by creating some badly named session variable. The limits of this patch (solved by other patches): - session variables block parallel execution - session variables blocks simple expression evaluation (in plpgsql) - SQL functions with session variables are not inlined - CALL statement is not supported (usage of direct access to express executor) - EXECUTE statement is not supported (usage of direct access to express executor) - memory used by dropped session variables is not released Implementations of EXPLAIN LET and PREPARE LET statements are in separate patches (for better readability)
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‎doc/src/sgml/catalogs.sgml

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@@ -9831,6 +9831,19 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>varcreate_lsn</structfield> <type>pg_lsn</type>
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</para>
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<para>
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LSN of the transaction where the variable was created.
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<structfield>varcreate_lsn</structfield> and
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<structfield>oid</structfield> together form the all-time unique
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identifier (<structfield>oid</structfield> alone is not enough, since
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object identifiers can get reused).
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</para></entry>
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</row>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>varname</structfield> <type>name</type>

‎doc/src/sgml/ddl.sgml

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@@ -5385,6 +5385,67 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate &gt;= DATE '2008-01-01';
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The session variable holds value in session memory. This value is private
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to each session and is released when the session ends.
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</para>
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<para>
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The value of a session variable is set with the SQL statement
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<command>LET</command>. The value of a session variable can be retrieved
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with the SQL statement <command>SELECT</command>.
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<programlisting>
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CREATE VARIABLE var1 AS date;
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LET var1 = current_date;
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SELECT var1;
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</programlisting>
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or
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<programlisting>
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CREATE VARIABLE public.current_user_id AS integer;
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GRANT SELECT ON VARIABLE public.current_user_id TO PUBLIC;
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LET current_user_id = (SELECT id FROM users WHERE usename = session_user);
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SELECT current_user_id;
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</programlisting>
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</para>
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<para>
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The value of a session variable is local to the current session. Retrieving
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a variable's value returns a <literal>NULL</literal>, unless its value has
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been set to something else in the current session using the
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<command>LET</command> command. Session variables are not transactional:
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any changes made to the value of a session variable in a transaction won't
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be undone if the transaction is rolled back (just like variables in
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procedural languages). Session variables themselves are persistent, but
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their values are neither persistent nor shared (like the content of
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temporary tables).
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</para>
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<para>
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Inside a query or an expression, a session variable can be
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<quote>shadowed</quote> by a column with the same name. Similarly, the
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name of a function or procedure argument or a PL/pgSQL variable (see
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<xref linkend="plpgsql-declarations"/>) can shadow a session variable
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in the routine's body. Such collisions of identifiers can be resolved
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by using qualified identifiers: Session variables can be qualified with
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the schema name, columns can use table aliases, routine variables can use
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block labels, and routine arguments can use the routine name.
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<programlisting>
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CREATE VARIABLE name AS text;
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LET name = 'John';
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CREATE TABLE foo(name text);
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INSERT INTO foo VALUES('Alice');
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-- variable name is shadowed
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SELECT name FROM foo;
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</programlisting>
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which returns:
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<programlisting>
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name
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-------
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Alice
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</programlisting>
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</para>
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</sect1>
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<sect1 id="ddl-others">

‎doc/src/sgml/func.sgml

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@@ -25860,8 +25860,8 @@ SELECT has_function_privilege('joeuser', 'myfunc(int, text)', 'execute');
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't' for <literal>TABLESPACE</literal>,
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'F' for <literal>FOREIGN DATA WRAPPER</literal>,
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'S' for <literal>FOREIGN SERVER</literal>,
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or
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'T' for <literal>TYPE</literal> or <literal>DOMAIN</literal>.
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'T' for <literal>TYPE</literal> or <literal>DOMAIN</literal> or
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'V' for <literal>SESSION VARIABLE</literal>.
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</para></entry>
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</row>
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‎doc/src/sgml/glossary.sgml

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<para>
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A persistent database object that holds a value in session memory. This
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value is private to each session and is released when the session ends.
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Read or write access to session variables is controlled by privileges,
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similar to other database objects.
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The default value of the session variable is null. Read or write access
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to session variables is controlled by privileges, similar to other database
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objects.
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</para>
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<para>
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For more information, see <xref linkend="ddl-session-variables"/>.

‎doc/src/sgml/parallel.sgml

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Plan nodes that reference a correlated <literal>SubPlan</literal>.
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</para>
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</listitem>
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<listitem>
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<para>
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Plan nodes that use a session variable.
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</para>
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</listitem>
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</itemizedlist>
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<sect2 id="parallel-labeling">

‎doc/src/sgml/plpgsql.sgml

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</programlisting>
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</para>
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</sect3>
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<sect3 id="plpgsql-porting-package-variables">
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<title><command>Packages and package variables</command></title>
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<para>
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The <application>PL/pgSQL</application> language has no packages, and
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therefore no package variables or package constants.
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You can consider translating an Oracle package into a schema in
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<productname>PostgreSQL</productname>. Package functions and procedures
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would then become functions and procedures in that schema, and package
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variables could be translated to session variables in that schema.
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(see <xref linkend="ddl-session-variables"/>).
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</para>
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</sect3>
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</sect2>
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<sect2 id="plpgsql-porting-appendix">

‎doc/src/sgml/ref/allfiles.sgml

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<!ENTITY grant SYSTEM "grant.sgml">
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<!ENTITY importForeignSchema SYSTEM "import_foreign_schema.sgml">
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<!ENTITY insert SYSTEM "insert.sgml">
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<!ENTITY let SYSTEM "let.sgml">
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<!ENTITY listen SYSTEM "listen.sgml">
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<!ENTITY load SYSTEM "load.sgml">
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<!ENTITY lock SYSTEM "lock.sgml">

‎doc/src/sgml/ref/alter_variable.sgml

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<simplelist type="inline">
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<member><xref linkend="sql-createvariable"/></member>
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<member><xref linkend="sql-dropvariable"/></member>
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<member><xref linkend="sql-let"/></member>
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</simplelist>
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</refsect1>
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</refentry>

‎doc/src/sgml/ref/create_variable.sgml

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<title>Examples</title>
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<para>
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Create an date session variable <literal>var1</literal>:
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Create a session variable <literal>var1</literal> of data type date:
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<programlisting>
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CREATE VARIABLE var1 AS date;
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LET var1 = current_date;
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SELECT var1;
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</programlisting>
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</para>
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<simplelist type="inline">
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<member><xref linkend="sql-altervariable"/></member>
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<member><xref linkend="sql-dropvariable"/></member>
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<member><xref linkend="sql-let"/></member>
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</simplelist>
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</refsect1>
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‎doc/src/sgml/ref/drop_variable.sgml

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<simplelist type="inline">
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<member><xref linkend="sql-altervariable"/></member>
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<member><xref linkend="sql-createvariable"/></member>
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<member><xref linkend="sql-let"/></member>
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</simplelist>
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</refsect1>
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‎doc/src/sgml/ref/let.sgml

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<!--
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doc/src/sgml/ref/let.sgml
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PostgreSQL documentation
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-->
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<refentry id="sql-let">
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<indexterm zone="sql-let">
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<primary>LET</primary>
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</indexterm>
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<indexterm>
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<primary>session variable</primary>
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<secondary>changing</secondary>
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</indexterm>
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<refmeta>
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<refentrytitle>LET</refentrytitle>
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<manvolnum>7</manvolnum>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>LET</refname>
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<refpurpose>change a session variable's value</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<synopsis>
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LET <replaceable class="parameter">session_variable</replaceable> = <replaceable class="parameter">sql_expression</replaceable>
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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The <command>LET</command> command assigns a value to the specified session
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variable.
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</para>
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</refsect1>
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<refsect1>
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<title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><replaceable class="parameter">session_variable</replaceable></term>
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<listitem>
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<para>
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The name of the session variable.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">sql_expression</replaceable></term>
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<listitem>
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<para>
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An arbitrary SQL expression. The result must be of a data type that can
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be cast to the type of the session variable in an assignment.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<programlisting>
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CREATE VARIABLE myvar AS integer;
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LET myvar = 10;
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LET myvar = (SELECT sum(val) FROM tab);
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</programlisting>
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</refsect1>
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<refsect1>
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<title>Compatibility</title>
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<para>
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The <command>LET</command> is a <productname>PostgreSQL</productname>
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extension.
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</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<simplelist type="inline">
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<member><xref linkend="sql-altervariable"/></member>
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<member><xref linkend="sql-createvariable"/></member>
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<member><xref linkend="sql-dropvariable"/></member>
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</simplelist>
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</refsect1>
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</refentry>

‎doc/src/sgml/reference.sgml

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&grant;
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&importForeignSchema;
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&insert;
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&let;
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&listen;
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&load;
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&lock;

‎src/backend/catalog/dependency.c

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{
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Param *param = (Param *) node;
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/* a variable parameter depends on the session variable */
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if (param->paramkind == PARAM_VARIABLE)
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add_object_address(VariableRelationId, param->paramvarid, 0,
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context->addrs);
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/* A parameter must depend on the parameter's datatype */
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add_object_address(TypeRelationId, param->paramtype, 0,
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context->addrs);

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