Thursday, May 23, 2013

Intelligent scan selectivity reduction: Sybase optimized index selection

Sybase ASE has a nice optimization feature called "Intelligent scan selectivity reduction", which optimizes the index selection, when the predicate columns do not include the prefix columns in the composite index.  It seems there's not much documentation about it. So I'll give a simple introduction about this hidden feature here.

This feature is only available for data page lock(DPL)/data row lock (DRL) tables, when the following conditions are met:
- the table has a composite index
- index prefix column is not in predicate
- cardinality of the prefix column is fairly low

Without this feature, we may need to do a full table scan, or create new index on the predicates.
With this feature, the optimizer will treat the composite index search as a number of small sub-indexes to retrieve the row IDs.
- determine the domain of distinct values for the index prefix column
- iterate through each distinct value in this domain
- for each distinct value, the optimizer will perform a regular index scan

The Intelligent Scan feature will function as if you issued a sequence of SQL statements having each statement specifying a single value for the index prefix column. Probing the table several times using index scan is often more efficient than performing a table scan.

A simple example:
create table t1(c1 int, c2 int, c3 varchar(10), c4 varchar(100)) lock datarow
go
create index t1_i1 on t1(c1, c2, c3)
go
select c4 from t1 where c2 = 2
go
If c1 only has 10 distinct values, then optimizer will still use index ti_i1. For each c1 value, it'll use t1_i1 to find the mathcing c2 values. For large table this is often better than a full table scan on t1.

You can always turn off this feature by using trace flag 15353.

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