Oracle Tips by Burleson

Collecting Statistics  

You can collect statistics about the physical storage characteristics and data distribution of an index, table, column, or cluster, and store them as histograms in the data dictionary. For computing or estimating statistics, computation always provides exact values but can take longer than estimation, and requires large amounts of temporary tablespace (up to four times the size of your largest table). Estimation is often much faster than computation, and the results are usually nearly exact. You cannot compute or estimate histogram statistics for the following column types:

•  REFs

•  varrays 

•  nested tables

•  LOBs

•  LONGs

•  object types

Use estimation, rather than computation, unless you feel you need exact values. Some statistics are always computed exactly, regardless of whether you specify computation or estimation. If you choose estimation, and the time saved by estimating a statistic is negligible, Oracle computes the statistic exactly.      

If the data dictionary already contains statistics for the analyzed object, Oracle updates the existing statistics with the new ones.

ANALYZE Myths

Since ANALYZE was introduced, many myths about its use have been circulated. Some of the more harmful ones are:

•  You need to ANALYZE entire schema.

•  ANALYZE does a full row count no matter what.

•  You can sample 5 to 10 percent and get good results.

•  You can sample 50 rows and get good results.

See Code Depot

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