Add comment on no default partition with hash partitioning

Discussion: https://postgr.es/m/20190806222735.GA9535@alvherre.pgsql

diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index a9b2f8bacd807d49241b2bd451a1bc903da7f1e6..6e5768c66cffc7bd039282cae44133730ef10c60 100644 (file)
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -3727,6 +3727,12 @@ transformPartitionBound(ParseState *pstate, Relation parent,
 
        if (spec->is_default)
        {
+               /*
+                * Hash partitioning does not support a default partition; there's no
+                * use case for it (since the set of partitions to create is perfectly
+                * defined), and if users do get into it accidentally, it's hard to
+                * back out from it afterwards.
+                */
                if (strategy == PARTITION_STRATEGY_HASH)
                        ereport(ERROR,
                                        (errcode(ERRCODE_INVALID_TABLE_DEFINITION),