Update statement taking too long to execute

Question

Hi All,
I'm trying to run this update statement. But its taking too long to execute.
    UPDATE ops_forecast_extract b SET position_id = (SELECT a.row_id
        FROM s_postn a
        WHERE UPPER(a.desc_text) = UPPER(TRIM(B.POSITION_NAME)))
        WHERE position_level = 7
        AND b.am_id IS NULL;
        SELECT COUNT(*) FROM S_POSTN;
        214665
        SELECT COUNT(*) FROM ops_forecast_extract;
        49366
SELECT count(*)
        FROM s_postn a, ops_forecast_extract b
        WHERE UPPER(a.desc_text) = UPPER(TRIM(B.POSITION_NAME));
575What could be the reason for update statement to execute so long?
Thanks

Answer 1

polasa wrote:
Hi All,
I'm trying to run this update statement. But its taking too long to execute.
What could be the reason for update statement to execute so long?You haven't said what "too long" means, but a simple reason could be that the scalar subquery on "s_postn" is using a full table scan for each execution. Potentially this subquery gets executed for each row of the "ops_forecast_extract" table that satisfies your filter predicates. "Potentially" because of the cunning "filter/subquery optimization" of the Oracle runtime engine that attempts to cache the results of already executed instances of the subquery. Since the in-memory hash table that holds these cached results is of limited size, the optimization algorithm depends on the sort order of the data and could suffer from hash collisions it's unpredictable how well this optimization works in your particular case.
You might want to check the execution plan, it should tell you at least how Oracle is going to execute the scalar subquery (it doesn't tell you anything about this "filter/subquery optimization" feature).
Generic instructions how to generate a useful explain plan output and how to post it here follow:
Could you please post an properly formatted explain plan output using DBMS_XPLAN.DISPLAY including the "Predicate Information" section below the plan to provide more details regarding your statement. Please use the {noformat}[{noformat}code{noformat}]{noformat} tag before and {noformat}[{noformat}/code{noformat}]{noformat} tag after or the {noformat}{{noformat}code{noformat}}{noformat} tag before and after to enhance readability of the output provided:
In SQL*Plus:
SET LINESIZE 130
EXPLAIN PLAN FOR <your statement>;
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);Note that the package DBMS_XPLAN.DISPLAY is only available from 9i on.
In 9i and above, if the "Predicate Information" section is missing from the DBMS_XPLAN.DISPLAY output but you get instead the message "Plan table is old version" then you need to re-create your plan table using the server side script "$ORACLE_HOME/rdbms/admin/utlxplan.sql".
In previous versions you could run the following in SQL*Plus (on the server) instead:
@?/rdbms/admin/utlxplsA different approach in SQL*Plus:
SET AUTOTRACE ON EXPLAIN
<run your statement>;will also show the execution plan.
In order to get a better understanding where your statement spends the time you might want to turn on SQL trace as described here:
When your query takes too long ...
and post the "tkprof" output here, too.
Regards,
Randolf
Oracle related stuff blog:
http://oracle-randolf.blogspot.com/
SQLTools++ for Oracle (Open source Oracle GUI for Windows):
http://www.sqltools-plusplus.org:7676/
http://sourceforge.net/projects/sqlt-pp/