Wednesday, August 8, 2007

choose index vs FTS(full table scan)

http://www.isug.com/Sybase_FAQ/ASE/section1.5.html#1.5.3 has a good story. Here's my version of the story, perhaps less accurate.

limited RAM => can't keep all data blocks in cache. frequent swap-out/page-out => index may point to Page 5 first, and 986 pages later, Page 5 again => same disk data-block retrieved twice => index solution may require more disk-reads than a FTS.

The simplified story above helps us focus on a small, bite-sized topic. When you feel ready, read the longer version below.

Besides, Remember index itself need to be retrieved from disk.
How to choose between index and FTS? [[ oracle sql tuning ]] and [[mysql stored procedure ]] both mention somewhere around 10% [1] as a threshold to switch to FTS.
[1] [[ oracle sql tuning ]] p 233 gives different thresholds for row-sequenced tables. Same chapters also mention the influence of parallel query.

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