My query was fine last week and now it is slow. Why? (for DBA
ORACLE Interview Questions and Answers (Part 3)
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385. My query was fine last week and now it is slow. Why? (for DBA
The likely cause of this is because the execution plan has changed. Generate a current explain plan of the offending query and compare it to a previous one that was taken when the query was performing well. Usually the previous plan is not available.
Some factors that can cause a plan to change are:
. Which tables are currently analyzed? Were they previously analyzed? (ie. Was the query using RBO and now CBO?)
. Has OPTIMIZER_MODE been changed in INIT.ORA?
. Has the DEGREE of parallelism been defined/changed on any table?
. Have the tables been re-analyzed? Were the tables analyzed using estimate or compute? If estimate, what percentage was used?
. Have the statistics changed?
. Has the INIT.ORA parameter DB_FILE_MULTIBLOCK_READ_COUNT been changed?
. Has the INIT.ORA parameter SORT_AREA_SIZE been changed?
. Have any other INIT.ORA parameters been changed?
. What do you think the plan should be? Run the query with hints to see if this produces the required performance.
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