ANALYZE (Part 1)

(Tested on an Oracle 10 database.)

I was going through some old notes recently and was reminded of the ANALYZE command. This allows you to calculate statistics on a table or index. The optimizer can then use these to improve the efficiency of subsequent DML on that object. Oracle have said for a long time now that you should DBMS_STATS in preference to ANALYZE. However, I thought it might still be useful to look at ANALYZE in case you find it in an old SQL script yourself.

In the example below, a table is created and its rows counted. At this point, NUM_ROWS (the number of rows in the table when it was last analyzed) is blank. SAMPLE_SIZE (the number of rows used for the last ANALYZE) is also blank. The table is then analyzed with the COMPUTE STATISTICS option. This analyzes the entire table and populates these two columns in the process.

The table is then analyzed with the ESTIMATE STATISTICS SAMPLE 100 ROWS option. You might expect SAMPLE_SIZE to be 100 afterwards but it is not. I will be returning to this in a future post.

The table is then analyzed with the ESTIMATE STATISTICS SAMPLE 20 PERCENT option. You might expect SAMPLE_SIZE to be 306 afterwards but it is not. I will be returning to this too in a future post.

Finally, the table is analyzed with the DELETE STATISTICS option. This removes the statistics so the NUM_ROWS and SAMPLE_SIZE columns become null again:

SQL> create table andrew
  2  as select * from dba_tables
  3  /

Table created.

SQL> select count(*) from andrew
  2  /

  COUNT(*)
----------
      1531

SQL> select num_rows, sample_size
  2  from dba_tables
  3  where table_name = 'ANDREW'
  4  /

  NUM_ROWS SAMPLE_SIZE
---------- -----------


SQL> analyze table andrew compute statistics
  2  /

Table analyzed.

SQL> select num_rows, sample_size
  2  from dba_tables
  3  where table_name = 'ANDREW'
  4  /

  NUM_ROWS SAMPLE_SIZE
---------- -----------
      1531        1531

SQL> analyze table andrew estimate statistics
  2  sample 100 rows
  3  /

Table analyzed.

SQL> select num_rows, sample_size
  2  from dba_tables
  3  where table_name = 'ANDREW'
  4  /

  NUM_ROWS SAMPLE_SIZE
---------- -----------
      1531        1531

SQL> analyze table andrew estimate statistics
  2  sample 20 percent
  3  /

Table analyzed.

SQL> select num_rows, sample_size
  2  from dba_tables
  3  where table_name = 'ANDREW'
  4  /

  NUM_ROWS SAMPLE_SIZE
---------- -----------
      1531        1531

SQL> analyze table andrew delete statistics
  2  /

Table analyzed.

SQL> select num_rows, sample_size
  2  from dba_tables
  3  where table_name = 'ANDREW'
  4  /

  NUM_ROWS SAMPLE_SIZE
---------- -----------


SQL>