| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: getting rid of SnapshotNow |
| Date: | 2013-07-23 18:24:32 |
| Message-ID: | 4668.1374603872@sss.pgh.pa.us |
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| Lists: | pgsql-hackers pgsql-odbc |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Jul 23, 2013 at 12:28 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> As far as get_actual_variable_range() is concerned, an MVCC snapshot
>> would probably be the thing to use anyway;
> That surprises me, though. I really thought the point was to cost the
> index scan, and surely that will be slowed down even by entries we
> can't see.
No, the usage (or the main usage anyway) is for selectivity estimation,
ie how many rows will the query fetch.
regards, tom lane
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