| From: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Bill Moran <wmoran(at)potentialtech(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com> |
| Subject: | Re: SELECT DISTINCT never uses an index? |
| Date: | 2016-07-07 22:08:18 |
| Message-ID: | CAKJS1f_7YLoO-MnJTRir9DfPOzNtaH=p=6+e8SteiP70ZcwyLg@mail.gmail.com |
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| Lists: | pgsql-hackers |
On 8 July 2016 at 09:49, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> We're probably missing a few tricks on queries of this type. If the
> index-traversal machinery had a mechanism to skip quickly to the next
> distinct value, that could be used here: walk up the btree until you
> find a page that contains keyspace not equal to the current key, then
> walk back down until you find the first leaf page that contains such a
> value. That would potentially let you step over large chunks of the
> index without actually examining all the leaf pages, which for a query
> like this seems like it could be a big win.
Thomas Munro did take some initial steps to implementing this a few years ago:
https://www.postgresql.org/message-id/CADLWmXXbTSBxP-MzJuPAYSsL_2f0iPm5VWPbCvDbVvfX93FKkw%40mail.gmail.com
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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