Re: Bitmap indexes etc.

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Ivan Voras <ivoras(at)fer(dot)hr>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Bitmap indexes etc.
Date: 2005-12-26 21:57:11
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Ivan Voras <ivoras(at)fer(dot)hr> writes:
> This is PostgreSQL 8.1.0.

> - what does "Bitmap Heap Scan" phase do?

A plain indexscan fetches one tuple-pointer at a time from the index,
and immediately visits that tuple in the table. A bitmap scan fetches
all the tuple-pointers from the index in one go, sorts them using an
in-memory "bitmap" data structure, and then visits the table tuples in
physical tuple-location order. The bitmap scan improves locality of
reference to the table at the cost of more bookkeeping overhead to
manage the "bitmap" data structure --- and at the cost that the data
is no longer retrieved in index order, which doesn't matter for your
query but would matter if you said ORDER BY.

> - what is "Recheck condition" and why is it needed?

If the bitmap gets too large we convert it to "lossy" style, in which we
only remember which pages contain matching tuples instead of remembering
each tuple individually. When that happens, the table-visiting phase
has to examine each tuple on the page and recheck the scan condition to
see which tuples to return.

> - why are proposed "width" fields in the plan different between the two
> plans?

Updated statistics about average column widths, presumably.

> (actually, a nice explanation what exactly are those widths would also
> be nice :) )

Sum of the average widths of the columns being fetched from the table.

> - I thought "Bitmap Index Scan" was only used when there are two or more
> applicable indexes in the plan, so I don't understand why is it used
> now?

True, we can combine multiple bitmaps via AND/OR operations to merge
results from multiple indexes before visiting the table ... but it's
still potentially worthwhile even for one index. A rule of thumb is
that plain indexscan wins for fetching a small number of tuples, bitmap
scan wins for a somewhat larger number of tuples, and seqscan wins if
you're fetching a large percentage of the whole table.

regards, tom lane

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