| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Richard Guo <riguo(at)pivotal(dot)io> |
| Cc: | PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Pulling up direct-correlated ANY_SUBLINK |
| Date: | 2019-09-10 13:48:48 |
| Message-ID: | 27733.1568123328@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Richard Guo <riguo(at)pivotal(dot)io> writes:
> Currently we do not try to pull up sub-select of type ANY_SUBLINK if it
> refers to any Vars of the parent query, as indicated in the code snippet
> below:
> if (contain_vars_of_level((Node *) subselect, 1))
> return NULL;
> Why do we have this check?
Because the result would not be a join between two independent tables.
> Can we try to pull up direct-correlated ANY SubLink with the help of
> LATERAL?
Perhaps. But what's the argument that you'd end up with a better
plan? LATERAL pretty much constrains things to use a nestloop,
so I'm not sure there's anything fundamentally different.
regards, tom lane
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