Improve docs and error messages for parallel vacuum.

The error messages, docs, and one of the options were using
'parallel degree' to indicate parallelism used by vacuum command. We
normally use 'parallel workers' at other places so change it for parallel
vacuum accordingly.

Author: Bharath Rupireddy
Reviewed-by: Dilip Kumar, Amit Kapila
Backpatch-through: 13
Discussion: https://postgr.es/m/CALj2ACWz=PYrrFXVsEKb9J1aiX4raA+UBe02hdRp_zqDkrWUiw@mail.gmail.com

Author: Amit Kapila

doc/src/sgml/ref/vacuumdb.sgml

@@ -230,11 +230,11 @@ PostgreSQL documentation
      </varlistentry>
 
      <varlistentry>
-      <term><option>-P <replaceable class="parameter">parallel_degree</replaceable></option></term>
-      <term><option>--parallel=<replaceable class="parameter">parallel_degree</replaceable></option></term>
+      <term><option>-P <replaceable class="parameter">parallel_workers</replaceable></option></term>
+      <term><option>--parallel=<replaceable class="parameter">parallel_workers</replaceable></option></term>
       <listitem>
        <para>
-        Specify the parallel degree of <firstterm>parallel vacuum</firstterm>.
+        Specify the number of parallel workers for <firstterm>parallel vacuum</firstterm>.
         This allows the vacuum to leverage multiple CPUs to process indexes.
         See <xref linkend="sql-vacuum"/>.
        </para>

src/backend/commands/vacuum.c

@@ -160,7 +160,7 @@ ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
 				if (nworkers < 0 || nworkers > MAX_PARALLEL_WORKER_LIMIT)
 					ereport(ERROR,
 							(errcode(ERRCODE_SYNTAX_ERROR),
-							 errmsg("parallel vacuum degree must be between 0 and %d",
+							 errmsg("parallel workers for vacuum must be between 0 and %d",
 									MAX_PARALLEL_WORKER_LIMIT),
 							 parser_errposition(pstate, opt->location)));
 

src/bin/scripts/vacuumdb.c

@@ -189,7 +189,7 @@ main(int argc, char *argv[])
 				vacopts.parallel_workers = atoi(optarg);
 				if (vacopts.parallel_workers < 0)
 				{
-					pg_log_error("parallel vacuum degree must be a non-negative integer");
+					pg_log_error("parallel workers for vacuum must be greater than or equal to zero");
 					exit(1);
 				}
 				break;
@@ -920,7 +920,7 @@ help(const char *progname)
 	printf(_("  -j, --jobs=NUM                  use this many concurrent connections to vacuum\n"));
 	printf(_("      --min-mxid-age=MXID_AGE     minimum multixact ID age of tables to vacuum\n"));
 	printf(_("      --min-xid-age=XID_AGE       minimum transaction ID age of tables to vacuum\n"));
-	printf(_("  -P, --parallel=PARALLEL_DEGREE  use this many background workers for vacuum, if available\n"));
+	printf(_("  -P, --parallel=PARALLEL_WORKERS use this many background workers for vacuum, if available\n"));
 	printf(_("  -q, --quiet                     don't write any messages\n"));
 	printf(_("      --skip-locked               skip relations that cannot be immediately locked\n"));
 	printf(_("  -t, --table='TABLE[(COLUMNS)]'  vacuum specific table(s) only\n"));

src/test/regress/expected/vacuum.out

@@ -110,7 +110,7 @@ VACUUM (PARALLEL 2) pvactst;
 UPDATE pvactst SET i = i WHERE i < 1000;
 VACUUM (PARALLEL 0) pvactst; -- disable parallel vacuum
 VACUUM (PARALLEL -1) pvactst; -- error
-ERROR:  parallel vacuum degree must be between 0 and 1024
+ERROR:  parallel workers for vacuum must be between 0 and 1024
 LINE 1: VACUUM (PARALLEL -1) pvactst;
                 ^
 VACUUM (PARALLEL 2, INDEX_CLEANUP FALSE) pvactst;