Re: Parallel INSERT (INTO ... SELECT ...)

On Fri, Oct 16, 2020 at 9:26 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
>
> Cool, let me try to explain my thoughts a bit more. The idea is first
> (in standard_planner) we check if there is any 'parallel_unsafe'
> function/expression (via max_parallel_hazard) in the query tree. If we
> don't find anything 'parallel_unsafe' then we mark parallelModeOk. At
> this stage, the patch is checking whether there is any
> 'parallel_unsafe' or 'parallel_restricted' expression/function in the
> target relation and if there is none then we mark parallelModeOK as
> true. So, if there is anything 'parallel_restricted' then we will mark
> parallelModeOK as false which doesn't seem right to me.
>
> Then later in the planner during set_rel_consider_parallel, we
> determine if a particular relation can be scanned from within a
> worker, then we consider that relation for parallelism. Here, we
> determine if certain things are parallel-restricted then we don't
> consider this for parallelism. Then we create partial paths for the
> relations that are considered for parallelism. I think we don't need
> to change anything for the current patch in these later stages because
> we anyway are not considering Insert to be pushed into workers.
> However, in the second patch where we are thinking to push Inserts in
> workers, we might need to do something to filter parallel-restricted
> cases during this stage of the planner.
>

Posting an update to the smaller patch (Parallel SELECT for INSERT
INTO...SELECT...).

Most of this patch feeds into the larger Parallel INSERT patch, for
which I'll also be posting an update soon.

Patch updates include:
- Removed explicit trigger-type checks (instead rely on declared
trigger parallel safety)
- Restored parallel-related XID checks that previous patch altered;
now assign XID prior to entering parallel-mode
- Now considers parallel-SELECT for parallel RESTRICTED cases (not
just parallel SAFE cases)
- Added parallel-safety checks for partition key expressions and
support functions
- Workaround added for test failure in "partition-concurrent-attach"
test; since ALTER TABLE operations may exclusively lock a relation
until end-of-transaction, now assume and return UNSAFE if can't
acquire a share-lock on the relation, rather than block until
potentially end of the other concurrent transaction in which the
exclusive lock is held.
Examples of when a relation is exclusively locked
(AccessExclusiveLock) until end-of-transaction include:
ALTER TABLE DROP COLUMN
ALTER TABLE ... RENAME
ALTER TABLE ... ATTACH PARTITION (locks default partition)

Regards,
Greg Nancarrow
Fujitsu Australia