Fix the usage of parallel and full options of vacuum command.

Earlier we were inconsistent in allowing the usage of parallel and
full options.  Change it such that we disallow them only when they are
combined in a way that we don't support.

In passing, improve the comments in some of the existing tests of parallel
vacuum.

Reported-by: Tushar Ahuja
Author: Justin Pryzby, Amit Kapila
Reviewed-by: Sawada Masahiko, Michael Paquier, Mahendra Singh Thalor and
Amit Kapila
Discussion: https://postgr.es/m/58c8d171-e665-6fa3-a9d3-d9423b694dae%40enterprisedb.com

doc/src/sgml/ref/vacuum.sgml

@@ -235,22 +235,22 @@ VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ <replaceable class="paramet
       Perform index vacuum and index cleanup phases of <command>VACUUM</command>
       in parallel using <replaceable class="parameter">integer</replaceable>
       background workers (for the details of each vacuum phase, please
-      refer to <xref linkend="vacuum-phases"/>).  If the
-      <literal>PARALLEL</literal> option is omitted, then the number of workers
-      is determined based on the number of indexes that support parallel vacuum
-      operation on the relation, and is further limited by <xref
-      linkend="guc-max-parallel-workers-maintenance"/>.
-      An index can participate in parallel vacuum if and only if the size
-      of the index is more than <xref linkend="guc-min-parallel-index-scan-size"/>.
-      Please note that it is not guaranteed that the number of parallel workers
-      specified in <replaceable class="parameter">integer</replaceable> will
-      be used during execution.  It is possible for a vacuum to run with fewer
-      workers than specified, or even with no workers at all.  Only one worker
-      can be used per index.  So parallel workers are launched only when there
-      are at least <literal>2</literal> indexes in the table.  Workers for
-      vacuum are launched before the start of each phase and exit at the end of
-      the phase.  These behaviors might change in a future release.  This
-      option can't be used with the <literal>FULL</literal> option.
+      refer to <xref linkend="vacuum-phases"/>).  In plain <command>VACUUM</command>
+      (without <literal>FULL</literal>), if the <literal>PARALLEL</literal> option
+      is omitted, then the number of workers is determined based on the number of
+      indexes on the relation that support parallel vacuum operation and is further
+      limited by <xref linkend="guc-max-parallel-workers-maintenance"/>.  An index
+      can participate in parallel vacuum if and only if the size of the index is
+      more than <xref linkend="guc-min-parallel-index-scan-size"/>.  Please note
+      that it is not guaranteed that the number of parallel workers specified in
+      <replaceable class="parameter">integer</replaceable> will be used during
+      execution.  It is possible for a vacuum to run with fewer workers than
+      specified, or even with no workers at all.  Only one worker can be used per
+      index.  So parallel workers are launched only when there are at least
+      <literal>2</literal> indexes in the table.  Workers for vacuum are launched
+      before the start of each phase and exit at the end of the phase.  These
+      behaviors might change in a future release.  This option can't be used with
+      the <literal>FULL</literal> option.
      </para>
     </listitem>
    </varlistentry>

src/backend/commands/vacuum.c

@@ -104,7 +104,6 @@ ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
 	bool		freeze = false;
 	bool		full = false;
 	bool		disable_page_skipping = false;
-	bool		parallel_option = false;
 	ListCell   *lc;
 
 	/* Set default value */
@@ -145,7 +144,6 @@ ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
 			params.truncate = get_vacopt_ternary_value(opt);
 		else if (strcmp(opt->defname, "parallel") == 0)
 		{
-			parallel_option = true;
 			if (opt->arg == NULL)
 			{
 				ereport(ERROR,
@@ -199,10 +197,10 @@ ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
 		   !(params.options & (VACOPT_FULL | VACOPT_FREEZE)));
 	Assert(!(params.options & VACOPT_SKIPTOAST));
 
-	if ((params.options & VACOPT_FULL) && parallel_option)
+	if ((params.options & VACOPT_FULL) && params.nworkers > 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-				 errmsg("cannot specify both FULL and PARALLEL options")));
+				 errmsg("VACUUM FULL cannot be performed in parallel")));
 
 	/*
 	 * Make sure VACOPT_ANALYZE is specified if any column lists are present.

src/test/regress/expected/vacuum.out

@@ -115,15 +115,17 @@ LINE 1: VACUUM (PARALLEL -1) pvactst;
                 ^
 VACUUM (PARALLEL 2, INDEX_CLEANUP FALSE) pvactst;
 VACUUM (PARALLEL 2, FULL TRUE) pvactst; -- error, cannot use both PARALLEL and FULL
-ERROR:  cannot specify both FULL and PARALLEL options
+ERROR:  VACUUM FULL cannot be performed in parallel
 VACUUM (PARALLEL) pvactst; -- error, cannot use PARALLEL option without parallel degree
 ERROR:  parallel option requires a value between 0 and 1024
 LINE 1: VACUUM (PARALLEL) pvactst;
                 ^
+-- Test different combinations of parallel and full options for temporary tables
 CREATE TEMPORARY TABLE tmp (a int PRIMARY KEY);
 CREATE INDEX tmp_idx1 ON tmp (a);
-VACUUM (PARALLEL 1) tmp; -- disables parallel vacuum option
+VACUUM (PARALLEL 1, FULL FALSE) tmp; -- parallel vacuum disabled for temp tables
 WARNING:  disabling parallel option of vacuum on "tmp" --- cannot vacuum temporary tables in parallel
+VACUUM (PARALLEL 0, FULL TRUE) tmp; -- can specify parallel disabled (even though that's implied by FULL)
 RESET min_parallel_index_scan_size;
 DROP TABLE pvactst;
 -- INDEX_CLEANUP option

src/test/regress/sql/vacuum.sql

@@ -100,9 +100,12 @@ VACUUM (PARALLEL -1) pvactst; -- error
 VACUUM (PARALLEL 2, INDEX_CLEANUP FALSE) pvactst;
 VACUUM (PARALLEL 2, FULL TRUE) pvactst; -- error, cannot use both PARALLEL and FULL
 VACUUM (PARALLEL) pvactst; -- error, cannot use PARALLEL option without parallel degree
+
+-- Test different combinations of parallel and full options for temporary tables
 CREATE TEMPORARY TABLE tmp (a int PRIMARY KEY);
 CREATE INDEX tmp_idx1 ON tmp (a);
-VACUUM (PARALLEL 1) tmp; -- disables parallel vacuum option
+VACUUM (PARALLEL 1, FULL FALSE) tmp; -- parallel vacuum disabled for temp tables
+VACUUM (PARALLEL 0, FULL TRUE) tmp; -- can specify parallel disabled (even though that's implied by FULL)
 RESET min_parallel_index_scan_size;
 DROP TABLE pvactst;