From: | TJ O'Donnell <tjo(at)acm(dot)org> |
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To: | pgsql-general(at)postgresql(dot)org |
Subject: | using new bitmap scans to index bit columns? |
Date: | 2005-11-09 18:18:35 |
Message-ID: | 43723D7B.8070906@acm.org |
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Lists: | pgsql-general |
I like the new bitmap scans and I'm wondering if there is any way
I can utilize them for my bitmask column, defined as bit(1024).
I use this column as a sort of fingerprint to quickly scan my tables.
But it is a scan, not an index. I have not figured out a way to
index the bitmask column. Is there some way it can be used as
an index now that there are bitmap scans in 8.1?
Currently I do this:
Select * from mytable where contains(bitmask, fingerprint(user_data))
and matches(datacolumn, user_data);
user_data is a string, like a regexp but with different semantics for chemical data.
bitmask is precomputed/stored as bit(1024) = fingerprint(datacolumn)
contains(a,b) returns bool as 'select b=(a&b);'
This works well because matches() is an expensive functions.
But it would work better if bitmask could be indexed, no?
TJ O'Donnell
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