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Re: Bitmap index thoughts

From: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
To: Jie Zhang <jzhang(at)greenplum(dot)com>
Cc: Gavin Sherry <swm(at)linuxworld(dot)com(dot)au>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Bitmap index thoughts
Date: 2006-12-27 10:58:55
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Lists: pgsql-hackers
Jie Zhang wrote:
> The "bitmap data segment" sounds good in terms of space. The problem is that
> one bitmap is likely to occupy more pages than before, which may hurt the
> query performance. 

We could have segments of say 1/5 of page. When a bitmap grows larger 
than that, the bitmap would be moved to a page of its own. That way we 
wouldn't get unnecessary fragmentation with large bitmaps, but small 
bitmaps would be stored efficiently.

> I have been thinking along the lines of increasing the
> number of last bitmap words stored in each LOV item, but not to occupy one
> page. This may prevent some cases Gavin indicated here, but not all.

That sounds like more special cases and complexity. I like the segment 
idea more.

But actually I'm not convinced we need to worry about efficient storage 
of small bitmaps at all. The typical use case for bitmap indexes is 
large tables with small number of distinct values, and the problem 
doesn't really arise in that scenario. Let's keep it simple for now, we 
can enhance it in later releases.

   Heikki Linnakangas

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