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[CF 1608] v20250515 - declarative session variables, LET command
This branch was automatically generated by a robot using patches from an email thread registered at: https://commitfest.postgresql.org/patch/1608 The branch will be overwritten each time a new patch version is posted to the thread, and also periodically to check for bitrot caused by changes on the master branch. Patch(es): https://www.postgresql.org/message-id/CAFj8pRCEjG005L=1vGbir-odYdcYxByOAeFVJ_K9eRcRF=MbNQ@mail.gmail.com Author(s): Pavel Stehule
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‎doc/src/sgml/catalogs.sgml

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@@ -369,6 +369,11 @@
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<entry><link linkend="catalog-pg-user-mapping"><structname>pg_user_mapping</structname></link></entry>
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<entry>mappings of users to foreign servers</entry>
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</row>
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<row>
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<entry><link linkend="catalog-pg-variable"><structname>pg_variable</structname></link></entry>
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<entry>session variables</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
@@ -3362,6 +3367,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
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<literal>T</literal> = type,
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<literal>n</literal> = schema,
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<literal>L</literal> = large object
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<literal>V</literal> = session variable
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</para></entry>
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</row>
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@@ -9779,4 +9785,132 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
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</table>
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</sect1>
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<sect1 id="catalog-pg-variable">
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<title><structname>pg_variable</structname></title>
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<indexterm zone="catalog-pg-variable">
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<primary>pg_variable</primary>
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</indexterm>
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<para>
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The catalog <structname>pg_variable</structname> stores information about
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session variables.
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</para>
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<table>
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<title><structname>pg_variable</structname> Columns</title>
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<tgroup cols="1">
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<thead>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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Column Type
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</para>
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<para>
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Description
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</para></entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry role="catalog_table_entry"><para role="column_definition">
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<structfield>oid</structfield> <type>oid</type>
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</para>
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<para>
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Row identifier
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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>vartype</structfield> <type>oid</type>
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(references <link linkend="catalog-pg-type"><structname>pg_type</structname></link>.<structfield>oid</structfield>)
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</para>
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<para>
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The OID of the variable's data type
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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>
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</para>
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<para>
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Name of the session variable
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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>varnamespace</structfield> <type>oid</type>
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(references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>oid</structfield>)
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</para>
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<para>
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The OID of the namespace that contains this variable
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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>varowner</structfield> <type>oid</type>
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(references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>oid</structfield>)
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</para>
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<para>
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Owner of the variable
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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>vartypmod</structfield> <type>int4</type>
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</para>
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<para>
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<structfield>vartypmod</structfield> records type-specific data
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supplied at variable creation time (for example, the maximum
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length of a <type>varchar</type> column). It is passed to
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type-specific input functions and length coercion functions.
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The value will generally be -1 for types that do not need <structfield>vartypmod</structfield>.
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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>varcollation</structfield> <type>oid</type>
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(references <link linkend="catalog-pg-collation"><structname>pg_collation</structname></link>.<structfield>oid</structfield>)
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</para>
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<para>
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The defined collation of the variable, or zero if the variable is
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not of a collatable data type.
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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>varacl</structfield> <type>aclitem[]</type>
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</para>
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<para>
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Access privileges; see
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<xref linkend="sql-grant"/> and
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<xref linkend="sql-revoke"/>
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for details
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</para></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</sect1>
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</chapter>

‎doc/src/sgml/config.sgml

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@@ -11374,6 +11374,131 @@ dynamic_library_path = '/usr/local/lib/postgresql:$libdir'
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</listitem>
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</varlistentry>
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<varlistentry id="guc-session-variables-ambiguity-warning" xreflabel="session_variables_ambiguity_warning">
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<term><varname>session_variables_ambiguity_warning</varname> (<type>boolean</type>)
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<indexterm>
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<primary><varname>session_variables_ambiguity_warning</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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When on, a warning is raised when any identifier in a query could be
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used as both a column identifier, routine variable or a session
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variable identifier. The default is <literal>off</literal>.
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</para>
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<para>
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Session variables can be shadowed by column references in a query, this
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is an expected behavior. Previously working queries shouldn't error out
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by creating any session variable, so session variables are always shadowed
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if an identifier is ambiguous. Variables should be referenced using
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anunambiguous identifier without any possibility for a collision with
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identifier of other database objects (column names or record fields names).
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The warning messages emitted when enabling <varname>session_variables_ambiguity_warning</varname>
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can help finding such identifier collision.
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<programlisting>
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CREATE TABLE foo(a int);
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INSERT INTO foo VALUES(10);
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CREATE VARIABLE a int;
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LET a = 100;
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SELECT a FROM foo;
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</programlisting>
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<screen>
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a
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----
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10
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(1 row)
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</screen>
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<programlisting>
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SET session_variables_ambiguity_warning TO on;
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SELECT a FROM foo;
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</programlisting>
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<screen>
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WARNING: session variable "a" is shadowed
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LINE 1: SELECT a FROM foo;
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^
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DETAIL: Session variables can be shadowed by columns, routine's variables and routine's arguments with the same name.
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a
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----
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10
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(1 row)
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</screen>
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</para>
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<para>
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This feature can significantly increase log size, so it's disabled by
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default. For testing or development environments it's recommended to
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enable it if you use session variables.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry id="guc-session-variables-use-fence-warning-guard" xreflabel="session_variables_use_fence_warning_guard">
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<term><varname>session_variables_use_fence_warning_guard</varname> (<type>boolean</type>)
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<indexterm>
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<primary><varname>session_variables_use_fence_warning_guard</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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When on, a warning is raised when a session variable identifier is used
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inside a query without variable fence. The default is <literal>off</literal>.
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The warning is raised only when variable is used in places, where an
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collisions with column names is possible.
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<programlisting>
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CREATE TABLE foo(a int);
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INSERT INTO foo VALUES(10);
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CREATE VARIABLE b int;
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LET b = 100;
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SELECT a, b FROM foo;
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</programlisting>
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<screen>
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a | b
11459+
----+-----
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10 | 100
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(1 row)
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</screen>
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<programlisting>
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SET session_variables_use_fence_warning_guard TO on;
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SELECT a, b FROM foo;
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</programlisting>
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<screen>
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WARNING: session variable "b" is not used inside variable fence
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LINE 1: SELECT a, b FROM foo;
11472+
^
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DETAIL: The collision of session variable' names and column names is possible.
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a | b
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----+-----
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10 | 100
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(1 row)
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</screen>
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<programlisting>
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SELECT a, VARIABLE(b) FROM foo;
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</programlisting>
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<screen>
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a | b
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----+-----
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10 | 100
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(1 row)
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</screen>
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</para>
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<para>
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This feature can significantly increase log size, so it's disabled by
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default. Unless another collision resolution technique is used
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(dedicated schema or using prefixes like <literal>_</literal>),
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the use of variable fence syntax is strongly recommended, and this
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warning should be enabled.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry id="guc-standard-conforming-strings" xreflabel="standard_conforming_strings">
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<term><varname>standard_conforming_strings</varname> (<type>boolean</type>)
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<indexterm><primary>strings</primary><secondary>standard conforming</secondary></indexterm>

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