Estimate Rows Returned by SQL

Question:  My application is an ad hoc query system and I want to estimate the number of rows returned by a SQL statement before running the SQL.  I know that this is the cardinality (rows returned estimates) for the SQL statement, but I need to know how to programmatically check the plan_table before running the query.  How can I estimate the size of the SQL result set? 

Answer:  To estimate the number of rows that the optimizer thinks that a query will return, you can use the explain plan syntax and read the plan_table for the cardinality column.  Each use in the application will need a plan_table:

SQL> CONN sys/password AS SYSDBA
Connected

SQL> @$ORACLE_HOME/rdbms/admin/utlxplan.sql

Table created.

SQL> GRANT ALL ON sys.plan_table TO public;

Grant succeeded.

SQL> CREATE PUBLIC SYNONYM plan_table FOR sys.plan_table;

Synonym created.

At this point, your application can populate the plan_table with the cardinality estimates:

SQL> explain plan set statement_id=’estimate_rows’ for select stuff from mmytab where myval = 12;  

Explained.  

Now, you can query the plan table.  You can modify this standard query to get the cardinality for the last step of the execution plan, which will display the estimated number of rows returned for the SQL statement:

select lpad(' ',2*level)||operation||' '||options||' '||object_name||
        decode(partition_start,NULL,NULL,
            ' PARTS('||partition_start||'-'||partition_stop||')')||
            decode(level,1,'  [Cost = '||Cost||']',Null) QUERY_PLAN,
        Object_Node, Other_Tag
from plan_table
where statement_id = 'estimate_rows'
connect by prior statement_id = statement_id and
            prior id = parent_id
start with id = 1
order by statement_id;

Please note:  The estimates for the number of rows returned will only be as current as your optimizer statistics as gathered by your dbms_stats utility.

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