| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Greg Stark <gsstark(at)mit(dot)edu> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: remove flatfiles.c |
| Date: | 2009-09-02 19:45:49 |
| Message-ID: | 25528.1251920749@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Greg Stark <gsstark(at)mit(dot)edu> writes:
> The backwards scan is awful for rotating media. The reading from the
> end and writing to the beginning is bad too, though hopefully the
> cache can help that.
Yeah. And all that pales in comparison to what happens in the indexes.
You have to insert index entries (retail) for each moved-in tuple,
then after doing the intermediate commit you run around and remove
the index entries for the moved-off tuples. Lots of nonsequential
access to insert the entries. The cleanup isn't so bad --- it's
comparable to what regular lazy VACUUM has to do --- but that's just
one step in a very expensive process.
regards, tom lane
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