INF: Indexes Use Distribution Page Statistics

  - Microsoft SQL Server version 4.2 for OS/2

SUMMARY

The following steps illustrate that distribution statistics are used when only a prefix of the index is used in the query.

MORE INFORMATION

1. Create a table and index as follows:

         create table t1 (c1 varchar(20), c2 varchar(20), c3 char(30) )
         create index i1 on t1 (c1,c2)
   

   The third column ensures that the table occupies enough pages to keep the optimizer from always choosing a table scan.

2. Populate the table with 200 rows of generated data with the following criteria:

   a. Generate all 200 values of c2 using a counter.

   b. Set c1=1 in the first 100 rows.

   c. Increment the value of c1 using a counter for the next 100 rows.

   With the table populated in this way, c1 will not be very selective below row 100; however, it will be very selective above 100.

3. Next, run update statistics and the following queries with SET SHOWPLAN ON:

         select * from t1 where c1=1
         select * from t1 where c1=150
   

   Note that the first query scans the table instead of using the index. This proves that the optimizer is aware that the index is very unselective when c1=1. In contrast, the second query uses the index. This indicates that the optimizer is also aware that the index is very selective when c1=150.