| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Narendra Pradeep U U <narendra(dot)pradeep(at)zohocorp(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Ambigous Plan - Larger Table on Hash Side |
| Date: | 2018-03-12 18:06:51 |
| Message-ID: | 14578.1520878011@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Andres Freund <andres(at)anarazel(dot)de> writes:
> Not sure I follow. Unless the values are equivalent (i.e. duplicate key
> values), why should non-uniformity in key space translate to hash space?
Duplicates are exactly the problem. See estimate_hash_bucket_stats.
> And if there's duplicates it shouldn't hurt much either, unless doing
> a semi/anti-join? All rows are going to be returned and IIRC we quite
> cheaply continue a bucket scan?
If the bucket containing the MCV is bigger than work_mem, you gotta
problem --- one not necessarily shared by the other relation.
regards, tom lane
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