| From: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
|---|---|
| To: | Teodor Sigaev <teodor(at)sigaev(dot)ru> |
| Cc: | Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Subject: | Re: Perfomance bug in v10 |
| Date: | 2017-06-01 23:47:42 |
| Message-ID: | CAKJS1f9hPst0+h6Ert09nSh8aWcYOwkYS-Gar7T8UGPxaq4dQA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On 2 June 2017 at 03:46, Teodor Sigaev <teodor(at)sigaev(dot)ru> wrote:
> I miss here why could the presence of index influence on that? removing
> index causes a good plan although it isn't used in both plans .
Unique indexes are used as proofs when deciding if a join to the
relation is "inner_unique". A nested loop unique join is costed more
cheaply than a non-unique one since we can skip to the next outer
tuple once we've matched the current outer tuple to an inner tuple. In
theory that's half as many comparisons for a non-parameterised nested
loop.
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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