One of the most important diagnostic tools in
troubleshooting inefficient SQL is the output of the EXPLAIN PLAN command. This
command loads a work table (called PLAN_TABLE by default) with the execution
plan steps that the Oracle Optimizer has calculated for a SQL statement. For

   SELECT empno, ename
   FROM emp
   WHERE ename LIKE 'S%';

Because the rows in the PLAN_TABLE form a hierarchy, queries
against it require the complicated START WITH and CONNECT BY clauses of the
SELECT statement. DBMS_XPLAN, introduced in Oracle 9i and expanded in 10g,
makes it much easier to format and display execution plans.

DBMS_XPLAN.DISPLAY is a table valued function that displays
the contents of a PLAN_TABLE. Unlike regular scalar functions like SUBSTR or
aggregate functions like SUM, table functions return a complete rowset. They are used in the FROM clause of the SELECT
statement, and must be preceded by the word TABLE. For example:


Listing A shows the results of the above EXPLAIN PLAN statement,
as formatted by DBMS_XPLAN.DISPLAY.

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What’s even more remarkable is that DBMS_XPLAN can also
display “live” execution plans from cursors stored within the System
Global Area (SGA).
Listing B shows
the same SQL statement executed by user SCOTT. By querying the V$SESSION view,
you can see the last SQL ID executed in SCOTT’s
session. This, in turn, can be fed into DBMS_XPLAN.DISPLAY_CURSOR to get the
execution plan used by that cursor.

Finally, the DBMS_XPLAN.DISPLAY_AWR function can be used to
look up an historical SQL statement captured in Oracle 10g‘s Automatic Workload Repository (AWR), and display
its execution plan. This gives you a seven-day rolling window of history that
you can access.

All three of these functions are powerful tools for tuning
your SQL statements.

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Bob Watkins (OCP, MCDBA, MCSE, MCT) is a computer professional with 25 years of experience as a technical trainer, consultant, and database administrator.