Re: Parallel Full Hash Join

Thomas Munro <thomas(dot)munro(at)gmail(dot)com> writes:
> I committed the main patch.

This left the following code in hash_inner_and_outer (joinpath.c):

/*
* If the joinrel is parallel-safe, we may be able to consider a
* partial hash join. However, we can't handle JOIN_UNIQUE_OUTER,
* because the outer path will be partial, and therefore we won't be
* able to properly guarantee uniqueness. Similarly, we can't handle
* JOIN_FULL and JOIN_RIGHT, because they can produce false null
* extended rows. Also, the resulting path must not be parameterized.
*/
if (joinrel->consider_parallel &&
save_jointype != JOIN_UNIQUE_OUTER &&
outerrel->partial_pathlist != NIL &&
bms_is_empty(joinrel->lateral_relids))
{

The comment is no longer in sync with the code: this if-test used to
reject JOIN_FULL and JOIN_RIGHT, and no longer does so, but the comment
still claims it should. Shouldn't we drop the sentence beginning
"Similarly"? (I see that there's now one sub-section that still rejects
such cases, but it no longer seems correct to claim that they're rejected
overall.)

regards, tom lane