Doc: mention executor memory usage for enable_partitionwise* GUCs

Prior to this commit, the docs for enable_partitionwise_aggregate and
enable_partitionwise_join mentioned the additional overheads enabling
these causes for the query planner, but they mentioned nothing about the
possible surge in work_mem-consuming executor nodes that could end up in
the final plan.  Dimitrios reported the OOM killer intervened on his
query as a result of using enable_partitionwise_aggregate=on.

Here we adjust the docs to mention the possible increase in the number of
work_mem-consuming executor nodes that can appear in the final plan as a
result of enabling these GUCs.

Reported-by: Dimitrios Apostolou
Reviewed-by: Ashutosh Bapat
Discussion: https://postgr.es/m/3603c380-d094-136e-e333-610914fb3e80%40gmx.net
Discussion: https://postgr.es/m/CAApHDvoZ0_yqwPFEpb6h261L76BUpmh5GxBQq0LeRzQ5Jh3zzg@mail.gmail.com
Backpatch-through: 12, oldest supported version

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 632b1747d6e209a3dd67d8d28583730d4841f64e..d35b82a582c54299a6bc49ac2958ce4de263f490 100644 (file)
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -5567,9 +5567,13 @@ ANY num_sync ( 
         joining the matching partitions.  Partitionwise join currently applies
         only when the join conditions include all the partition keys, which
         must be of the same data type and have one-to-one matching sets of
-        child partitions.  Because partitionwise join planning can use
-        significantly more CPU time and memory during planning, the default is
-        off.
+        child partitions.  With this setting enabled, the number of nodes
+        whose memory usage is restricted by work_mem
+        appearing in the final plan can increase linearly according to the
+        number of partitions being scanned.  This can result in a large
+        increase in overall memory consumption during the execution of the
+        query.  Query planning also becomes significantly more expensive in
+        terms of memory and CPU.  The default value is off.
        
       
      
@@ -5587,9 +5591,13 @@ ANY num_sync ( 
         tables to be performed separately for each partition.  If the
         GROUP BY clause does not include the partition
         keys, only partial aggregation can be performed on a per-partition
-        basis, and finalization must be performed later.  Because
-        partitionwise grouping or aggregation can use significantly more CPU
-        time and memory during planning, the default is
+        basis, and finalization must be performed later.  With this setting
+        enabled, the number of nodes whose memory usage is restricted by
+        work_mem appearing in the final plan can increase
+        linearly according to the number of partitions being scanned.  This
+        can result in a large increase in overall memory consumption during
+        the execution of the query.  Query planning also becomes significantly
+        more expensive in terms of memory and CPU.  The default value is
         off.