Re: Add SPLIT PARTITION/MERGE PARTITIONS commands

On Thu, Apr 11, 2024 at 10:20:53PM -0400, Robert Haas wrote:
> On Thu, Apr 11, 2024 at 9:54 PM Alexander Korotkov <aekorotkov@gmail.com> wrote:
> > I think we shouldn't unconditionally copy schema name and
> > relpersistence from the parent table.  Instead we should throw the
> > error on a mismatch like CREATE TABLE ... PARTITION OF ... does.  I'm
> > working on revising this fix.
> 
> We definitely shouldn't copy the schema name from the parent table. It
> should be possible to schema-qualify the new partition names, and if
> you don't, then the search_path should determine where they get
> placed.

+1.  Alexander Lakhin reported an issue with schemas and SPLIT, and I
noticed an issue with schemas with MERGE.  The issue I hit is occurs
when MERGE'ing into a partition with the same name, and it's fixed like
so:

--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -21526,8 +21526,7 @@ ATExecMergePartitions(List **wqueue, AlteredTableInfo *tab, Relation rel,
     {
         /* Create partition table with generated temporary name. */
         sprintf(tmpRelName, "merge-%u-%X-tmp", RelationGetRelid(rel), MyProcPid);
-        mergePartName = makeRangeVar(get_namespace_name(RelationGetNamespace(rel)),
-                                     tmpRelName, -1);
+        mergePartName = makeRangeVar(mergePartName->schemaname, tmpRelName, -1);
     }
     createPartitionTable(mergePartName,
                          makeRangeVar(get_namespace_name(RelationGetNamespace(rel)),

> One of the things I dislike about this type of feature -- not this
> implementation specifically, but just this kind of idea in general --
> is that the syntax mentions a whole bunch of tables but in a way where
> you can't set their properties. Persistence, reloptions, whatever.
> There's just no place to mention any of that stuff - and if you wanted
> to create a place, you'd have to invent special syntax for each
> separate thing. That's why I think it's good that the normal way of
> creating a partition is CREATE TABLE .. PARTITION OF. Because that
> way, we know that the full power of the CREATE TABLE statement is
> always available, and you can set anything that you could set for a
> table that is not a partition.

Right.  The current feature is useful and will probably work for 90% of
people's partitioned tables.

Currently, CREATE TABLE .. PARTITION OF does not create stats objects on
the child table, but MERGE PARTITIONS does, which seems strange.
Maybe stats should not be included on the new child ?

Note that stats on parent table are not analagous to indexes -
partitioned indexes do nothing other than cause indexes to be created on
any new/attached partitions.  But stats objects on the parent 1) cause
extended stats to be collected and computed across the whole partition
heirarchy, and 2) do not cause stats to be computed for the individual
partitions.

Partitions can have different column definitions, for example null
constraints, FKs, defaults.  And currently, if you MERGE partitions,
those will all be lost (or rather, replaced by whatever LIKE parent
gives).  I think that's totally fine - anyone using different defaults
on child tables could either not use MERGE PARTITIONS, or fix up the
defaults afterwards.  There's not much confusion that the details of the
differences between individual partitions will be lost when the
individual partitions are merged and no longer exist.
But I think it'd be useful to document how the new partitions will be
constructed.

-- 
Justin