Oracle num_rows in dba_tables Oracle Database Tips by Donald Burleson

One of the most common problems (prior to Oracle 10g) was that Oracle DBA's would accidentally change the execution behavior of their SQL by changing the num_rows for a table.  Sometimes a DBA wants to know the row count, and the data dictionary view dba_tables has a column named num_rows.

However, num_rows was not put there to help the DBA create management reports for his end-user managers!  The num_rows value was created to assist the cost-based SQL optimizer in determining the optimal execution plan for any SQL that accesses the table.  The num_rows column of the dba_tables view can be set with any of these commands:

analyze table FRED;

EXEC DBMS_UTILITY.analyze_schema('OWNER','FRED');

EXEC DBMS_STATS.gather_table_stats('OWNER', 'FRED');

When tuning SQL for a whole database, you can adjust many of the Oracle optimizer parameters, but for tuning an individual SQL statement, there is nothing better than changing the schema statistics.  Small able can appear to be large, and this will give you important clues about how num_rows is used to optimize your query execution.  In this example, we adjust num_rows to one million, making a small table appear large to the Oracle optimizer.  In general a small num_rows will increase the probability that the CBO will choose an index (as opposed to a full-table scan).

exec dbms_stats.set_table_stats(OWNNAME=>'FLINTSTONE', TABNAME=>'FRED',
NUMROWS=>100000000);

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