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Don Burleson Blog 







Inserts become slow as table grows

Oracle Database Tips by Donald BurlesonJuly 29, 2015

Question:  I have a insert job that runs hourly.  At first, I was seeing an insert rate of 3,000 rows per minute, but now that the table has over a million rows, the insert rate has slowed to a crawl.  It seems like as the table grows larger, the inserts run slower.  What can cause a slowdown in insert speed as a table grows?

Answer:  Oracle inserts can slow down for a multitude of reasons as a table grows. First, see my notes on Oracle insert performance tuning.   Here are the most common reasons for slowness of inserts:

  • Stale statistics - Do this first, re-analyze the table and ALL indexes on the table:

EXEC DBMS_STATS.gather_table_stats('SCOTT', 'EMPLOYEES');
EXEC DBMS_STATS.gather_index_stats('SCOTT', 'EMPLOYEES_PK');

  • Freelist issues - Tables that re-user free blocks can see issues with the overhead of re-using data blocks.  To avoid this, use INSERT APPEND (only of you are doing an insert without a VALUES clause).
  • NEXT extent size - If you have a tint NEXT extent size, you may see delays each time that Oracle extends the table and index.  It's best of you pre-allocate the space.
  • Index fragmentation - In rare cases, fragmented indexes can contribute to slow insert performance.  To see, try rebuilding your indexes.

Also, you might have issues with contention for multiple concurrent inserts, and check to see if you have ASSM contention (ASSM does not always perform well under very high concurrent DML loads).  If so, redefine the table with enough freelists and freelist groups for the target tables and associated indexes. 



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