Fix search_path to a safe value during maintenance operations.

While executing maintenance operations (ANALYZE, CLUSTER, REFRESH
MATERIALIZED VIEW, REINDEX, or VACUUM), set search_path to
'pg_catalog, pg_temp' to prevent inconsistent behavior.

Functions that are used for functional indexes, in index expressions,
or in materialized views and depend on a different search path must be
declared with CREATE FUNCTION ... SET search_path='...'.

This change was previously committed as 05e1737351, then reverted in
commit 2fcc7ee7af because it was too late in the cycle.

Preparation for the MAINTAIN privilege, which was previously reverted
due to search_path manipulation hazards.

Discussion: https://postgr.es/m/d4ccaf3658cb3c281ec88c851a09733cd9482f22.camel@j-davis.com
Discussion: https://postgr.es/m/E1q7j7Y-000z1H-Hr%40gemulon.postgresql.org
Discussion: https://postgr.es/m/e44327179e5c9015c8dda67351c04da552066017.camel%40j-davis.com
Reviewed-by: Greg Stark, Nathan Bossart, Noah Misch

doc/src/sgml/amcheck.sgml

@@ -24,6 +24,9 @@
   to hold, we can expect binary searches on the affected page to
   incorrectly guide index scans, resulting in wrong answers to SQL
   queries.  If the structure appears to be valid, no error is raised.
+  While these checking functions are run, the <xref
+  linkend="guc-search-path"/> is temporarily changed to <literal>pg_catalog,
+  pg_temp</literal>.
  </para>
  <para>
   Verification is performed using the same procedures as those used by

doc/src/sgml/brin.sgml

@@ -95,7 +95,9 @@
   </para>
 
   <para>
-   Lastly, the following functions can be used:
+   Lastly, the following functions can be used (while these functions run,
+   <xref linkend="guc-search-path"/> is temporarily changed to
+   <literal>pg_catalog, pg_temp</literal>):
    <simplelist>
     <member>
      <function>brin_summarize_new_values(regclass)</function>

doc/src/sgml/ref/analyze.sgml

@@ -205,6 +205,12 @@ ANALYZE [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] [ <r
    (This will not be sufficient if there is heavy update activity.)
   </para>
 
+  <para>
+   While <command>ANALYZE</command> is running, the <xref
+   linkend="guc-search-path"/> is temporarily changed to <literal>pg_catalog,
+   pg_temp</literal>.
+  </para>
+
   <para>
    <command>ANALYZE</command>
    requires only a read lock on the target table, so it can run in

doc/src/sgml/ref/cluster.sgml

@@ -153,6 +153,12 @@ CLUSTER [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] [ <r
     information.
    </para>
 
+   <para>
+    While <command>CLUSTER</command> is running, the <xref
+    linkend="guc-search-path"/> is temporarily changed to <literal>pg_catalog,
+    pg_temp</literal>.
+   </para>
+
    <para>
     When an index scan is used, a temporary copy of the table is created that
     contains the table data in the index order.  Temporary copies of each

doc/src/sgml/ref/create_index.sgml

@@ -789,6 +789,12 @@ Indexes:
    the table to generate statistics for these indexes.
   </para>
 
+  <para>
+   While <command>CREATE INDEX</command> is running, the <xref
+   linkend="guc-search-path"/> is temporarily changed to <literal>pg_catalog,
+   pg_temp</literal>.
+  </para>
+
   <para>
    For most index methods, the speed of creating an index is
    dependent on the setting of <xref linkend="guc-maintenance-work-mem"/>.

doc/src/sgml/ref/refresh_materialized_view.sgml

@@ -98,6 +98,12 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
    will be ordered that way; but <command>REFRESH MATERIALIZED
    VIEW</command> does not guarantee to preserve that ordering.
   </para>
+
+  <para>
+   While <command>REFRESH MATERIALIZED VIEW</command> is running, the <xref
+   linkend="guc-search-path"/> is temporarily changed to <literal>pg_catalog,
+   pg_temp</literal>.
+  </para>
  </refsect1>
 
  <refsect1>

doc/src/sgml/ref/reindex.sgml

@@ -291,6 +291,12 @@ REINDEX [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] { DA
    into expensive sequential scans.
   </para>
 
+  <para>
+   While <command>REINDEX</command> is running, the <xref
+   linkend="guc-search-path"/> is temporarily changed to <literal>pg_catalog,
+   pg_temp</literal>.
+  </para>
+
   <para>
    Reindexing a single index or table requires being the owner of that
    index or table.  Reindexing a schema or database requires being the

doc/src/sgml/ref/vacuum.sgml

@@ -443,6 +443,12 @@ VACUUM [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] [ <re
     does not have permission to vacuum.
    </para>
 
+   <para>
+    While <command>VACUUM</command> is running, the <xref
+    linkend="guc-search-path"/> is temporarily changed to <literal>pg_catalog,
+    pg_temp</literal>.
+   </para>
+
    <para>
     <command>VACUUM</command> cannot be executed inside a transaction block.
    </para>