Re: OT: postgres 8.1 features ... possible pressure on migration of features from enterprise edition down to standard edition?
- From: Niall Litchfield <niall.litchfield@xxxxxxxxx>
- To: bdbafh@xxxxxxxxx
- Date: Wed, 9 Nov 2005 13:32:37 +0000
On 11/8/05, Paul Drake <bdbafh@xxxxxxxxx> wrote:
>
> http://www.postgresql.org/docs/8.1/interactive/release.html#RELEASE-8-1
>
> The following seemed interesting to me.
>
> block quote:
>
> Allow index scans to use an intermediate in-memory bitmap (Tom)
>
> In previous releases, only a single index could be used to do lookups
> on a table. With this feature, if a query has WHERE tab.col1 = 4 and
> tab.col2 = 9, and there is no multicolumn index on col1 and col2, but
> there is an index on col1 and another on col2, it is possible to
> search both indexes and combine the results in memory, then do heap
> fetches for only the rows matching both the col1 and col2
> restrictions. This is very useful in environments that have a lot of
> unstructured queries where it is impossible to create indexes that
> match all possible access conditions. Bitmap scans are useful even
> with a single index, as they reduce the amount of random access
> needed; a bitmap index scan is efficient for retrieving fairly large
> fractions of the complete table, whereas plain index scans are not.
isn't this BITMAP CONVERSION (TO|FROM ROWIDS) which I believe can already
occur on regular indexes in SE (though I can't find a repeatable example
anywhere) when Oracle decides to convert use two different index access
paths, convert them to bitmaps and do a bitmap AND|OR and then do a bitmap
to ROWID conversion back again, rather than being an implementation of
bitmap indexes in SE.
"Allow nonconsecutive index columns to be used in a multicolumn index (Tom)
>
> For example, this allows an index on columns a,b,c to be used in a
> query with WHERE a = 4 and c = 10. "
Again index skip scan already happens in 9iSE, this sounds like much the
same thing
On a more humorous note, here is one feature that I'm glad Oracle does
> not support:
>
> "Make REINDEX DATABASE reindex all indexes in the database ".
Might be a damn useful command for MSSQL though.
--
Niall Litchfield
Oracle DBA
http://www.niall.litchfield.dial.pipex.com
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