RE: Delete followed by Select Count(1) - SLOW

Truncate basically resets the table's highwater mark to 0. Deletes don't do that. A 'select count(*)' does a full table scan, which much touch all the blocks up to the HWM, and that's what's taking the time. The REUSE STORAGE option keeps the extents allocated to the table, but the HWM is still reset to 0.

At least, that's what I was taught!

HTH,
Mike

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===========================================================================
Michael P. Vergara
Oracle DBA
Guidant Corporation


(909) 914-2304



-----Original Message-----
Sent: Thursday, August 16, 2001 2:33 PM
To: Multiple recipients of list ORACLE-L


I have a user that deleted all of the rows in a table


(i.e. 100,000), waited for it to complete, and then

ran a SELECT COUNT(1) FROM <table>. It took a few
minutes for '0 rows' to be returned to the prompt. The
table has ~60 extents (128k ea.). Granted, the number
of extents is excessive but it's a development
instance and this table is an exception.

Is Oracle scanning through all of the blocks, since
the space wasn't released, and this is the cause of
the latency?

The curious thing is that I told this user to use
TRUNCATE instead and we talked about using the
drop/reuse storage clauses. He performed a
TRUNCATE...REUSE STORAGE and the same select and it
was night and day in terms of performance. If the
allocated space isn't being released in this case
also, why is there such a performance difference
between the two?

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-- 
Author: Vergara, Michael (TEM)
  INET: mvergara_at_guidant.com

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