Change the docs for PARALLEL option of Vacuum.

The rules to choose the number of parallel workers to perform parallel
vacuum operation were not clearly specified.

Reported-by: Peter Eisentraut
Author: Amit Kapila
Backpatch-through: 13, where it was introduced
Discussion: https://postgr.es/m/36aa8aea-61b7-eb3c-263b-648e0cb117b7@2ndquadrant.com

diff --git a/doc/src/sgml/ref/vacuum.sgml b/doc/src/sgml/ref/vacuum.sgml
index 26ede69bb31d9f904be6d55bdc06084de2f4479c..21ab57d880489e536918821371c34dcc0b9d75c8 100644 (file)
--- a/doc/src/sgml/ref/vacuum.sgml
+++ b/doc/src/sgml/ref/vacuum.sgml
@@ -235,22 +235,22 @@ VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ 
       Perform index vacuum and index cleanup phases of VACUUM
       in parallel using integer
       background workers (for the details of each vacuum phase, please
-      refer to ).  In plain VACUUM
-      (without FULL), if the PARALLEL 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 .  An index
-      can participate in parallel vacuum if and only if the size of the index is
-      more than .  Please note
-      that it is not guaranteed that the number of parallel workers specified in
-      integer 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
-      2 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 FULL option.
+      refer to ).  The number of workers used
+      to perform the operation is equal to the number of indexes on the
+      relation that support parallel vacuum which is limited by the number of
+      workers specified with PARALLEL option if any which is
+      further limited by .
+      An index can participate in parallel vacuum if and only if the size of the
+      index is more than .
+      Please note that it is not guaranteed that the number of parallel workers
+      specified in integer 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 2 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 FULL option.