I was looking at a very simple statement as shown below to see if it can be
further optimized. This is a standard Oracle E-Business statement. The database
version is 11.2.0.4:
DELETE FROM MTL_SYSTEM_ITEMS_INTERFACE
WHERE
SEGMENT1 = :B1
;
Column SEGMENT1 is the second column of a two-column index,
MTL_SYSTEM_ITEMS_INTERFACE_N4. The original plan shows INDEX SKIP SCAN of the
index:
The original plan is shown below:
Rows (1st) Rows (avg) Rows (max) Row Source Operation
---------- ---------- ----------
---------------------------------------------------
0 0 0 DELETE MTL_SYSTEM_ITEMS_INTERFACE (cr=676
pr=457 pw=0 time=761695 us)
806 806 806 INDEX SKIP SCAN
MTL_SYSTEM_ITEMS_INTERFACE_N4 (cr=640 pr=0 pw=0 time=6248 us cost=277 size=6930
card=33)(object id 1989058)
I decided to create a separate index just on the SEGMENT1 column,
MTL_SYSTEM_ITEMS_INTERFACE_T, to see if that would improve the response time.
The explain plan is shown below
Rows (1st) Rows (avg) Rows (max) Row Source Operation
---------- ---------- ----------
---------------------------------------------------
0 0 0 DELETE MTL_SYSTEM_ITEMS_INTERFACE (cr=3 pr=0
pw=0 time=37 us)
806 806 0 INDEX RANGE SCAN
MTL_SYSTEM_ITEMS_INTERFACE_T (cr=3 pr=0 pw=0 time=12 us cost=1 size=2743
card=13)(object id 7755594)
There is a marked improvement in the cost of index scan when doing regular
index scan. We normally do not create custom indexes on standard Oracle tables
but since this statement is part of a one-time conversion run that we are
trying to optimize to fit in the allotted outage window, we can create this
index temporarily and then drop it after the conversion run.
I am trying to understand why the cost associated with the SKIP SCAN would be
that much different than a regular scan.
Thanks,
Amir
