Re: Problem with default partition pruning

At Mon, 8 Apr 2019 16:57:35 +0900, "Yuzuko Hosoya" <hosoya.yuzuko@lab.ntt.co.jp> wrote in <00c101d4ede0$babd4390$3037cab0$@lab.ntt.co.jp>
> > BTW, now I'm a bit puzzled between whether this case should be fixed by hacking on partprune.c like
> > this patch does or whether to work on getting the other patch committed and expect users to set
> > constraint_exclusion = on for this to behave as expected.  The original intention of setting
> > partition_qual in set_relation_partition_info() was for partprune.c to use it to remove useless
> > arguments of OR clauses which otherwise would cause the failure to correctly prune the default partitions
> > of sub-partitioned tables.  As shown by the examples in this thread, the original effort was
> > insufficient, which this patch aims to improve.  But, it also expands the scope of partprune.c's usage
> > of partition_qual, which is to effectively perform full-blown constraint exclusion without being
> > controllable by constraint_exclusion GUC, which may be seen as being good or bad.  The fact that it
> > helps in getting partition pruning working correctly in more obscure cases like those discussed in
> > this thread means it's good maybe.
> > 
> Umm, even though this modification might be overhead, I think this problem should be solved
> without setting constraint_exclusion GUC. But I'm not sure.

Partition pruning and constraint exclusion are orthogonal
functions. Note that the default value of the variable is
CONSTRAINT_EXCLUSION_PARTITION and the behavior is not a perfect
bug.  So I think we can reasonably ignore constraints when
constraint_exclusion is intentionally turned off.

As the result I propose to move the "if(partconstr)" block in the
latest patches after the constant-false block, changing the
condition as "if (partconstr && constraint_exclusion !=
CONSTRAINT_EXCLUSION_OFF)".

This make partprune reacts to constraint_exclusion the consistent
way with the old-fashioned partitioning.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center