Re: Add SPLIT PARTITION/MERGE PARTITIONS commands

On Tue, Aug 27, 2024 at 2:24 PM Dmitry Koval <d(dot)koval(at)postgrespro(dot)ru> wrote:
> They contains changes from reverted commits 1adf16b8fb, 87c21bb941, and
> subsequent fixes and improvements including df64c81ca9, c99ef1811a,
> 9dfcac8e15, 885742b9f8, 842c9b2705, fcf80c5d5f, 96c7381c4c, f4fc7cb54b,
> 60ae37a8bc, 259c96fa8f, 449cdcd486, 3ca43dbbb6, 2a679ae94e, 3a82c689fd,
> fbd4321fd5, d53a4286d7, c086896625, 4e5d6c4091.
> I didn't include fix 04158e7fa3 into patches because Robert Haas
> objected to its use.

To be clear, I'm not against 04158e7fa3. I just don't think it fixes everything.

> 1. Function createPartitionTable() should be rewritten using partitioned
> table OID (not name) and without using ProcessUtility().

Agree.

> 2. Should it be considered an error when we split a partition owned by
> another user and get partitions that owned by our user?
> (I think this is not a problem. Perhaps disallow merging other users'
> partitions would be too strict a restriction.)
>
> 3. About the functional index "create index on foo (run_me(a));".
> (Should we disallow merging of another user's partitions when
> partitioned table has functional indexes? SECURITY_RESTRICTED_OPERATION?)
>
> 4. Need to decide what is correct in case there are per-partition
> constraints or triggers on a split partition. They not duplicated to the
> new partitions now. (But might be in this case we should have an error
> or warning?)

I think we want to avoid giving errors or warnings. For all of these
cases, and others, we need to consider what the expected behavior is,
and have test cases and documentation as appropriate. But we shouldn't
think of it as "let's make it fail if the user does something that's
not safe" but rather "let's figure out how to make it safe."

> 5. "If we're merging partitions, wouldn't it be better to adjust the
> constraints on the first partition - or perhaps the largest partition if
> we want to be clever -- and insert the data from all of the others into
> it? Maybe that would even have syntax that puts the user in control of
> which partition survives, e.g. ALTER TABLE tab1 MERGE PARTITION part1
> WITH part2, part3, .... That would also make it really obvious to the
> user what all of the properties of part1 will be after the merge: they
> will be exactly the same as they were before the merge, except that the
> partition constraint will have been adjusted."
> (Similar optimization was proposed in [3] but was rejected [4]).

Interesting. Maybe it would be a good idea to set up some test cases
to see which approach is better in different cases. Like try moving
data from foo1 to foo2 with DELETE..INSERT vs. creating a new table
with CTAS from foo1 UNION ALL foo2 and then indexing it. I think
Alexander has a good point there, but I think my point is good too so
I'm not sure which way wins.

--
Robert Haas
EDB: http://www.enterprisedb.com