Expensive plan with join-predicate push-down (JPPD) gets chosen
Date: Thu, 6 Dec 2018 14:28:13 +0000
Message-ID: <99761_1544106508_5C09320C_99761_6127_1_41c2088667c84b839679592ffd2347b3_at_vontobel.com>
The optimizer generates an expensive JPPD plan on a 12.2 database. In contrast, I'm getting a much cheaper plan with "_push_join_predicate"=false.
It's a complex query, so I couldn't build a model so far, but I can reproduce the bad plan at will.
So, I'm going to describe the problem by using the information from the CBO (10053) trace.
First, the optimizer calculates the cost of applying JPPD to three different query blocks - annotated as: #10, #11 and #12 :
JPPD: Checking validity of push-down in query block SET$BADDE9FB_1 (#4)
JPPD: Checking validity of push-down from query block SET$BADDE9FB_1 (#4) to query block SEL$2DE89145 (#10)
Check Basic Validity for Non-Union View for query block SEL$2DE89145 (#10)
JPPD: Passed validity checks
JPPD: Checking validity of push-down from query block SET$BADDE9FB_1 (#4) to query block SEL$41289C4A (#11)
Check Basic Validity for Non-Union View for query block SEL$41289C4A (#11)
JPPD: Passed validity checks
JPPD: Checking validity of push-down from query block SET$BADDE9FB_1 (#4) to query block SEL$A0D20652 (#12)
Check Basic Validity for Non-Union View for query block SEL$A0D20652 (#12)
JPPD: Passed validity checks
JPPD: JPPD: Pushdown from query block SET$BADDE9FB_1 (#4) passed validity checks.
Join-Predicate push-down on query block SET$BADDE9FB_1 (#4)
JPPD: Using search type: linear JPPD: Considering join predicate push-down JPPD: Starting iteration 1, state space = (10,11,12) : (0,0,0) JPPD: Updated best state, Cost = 41463.286373 JPPD: Starting iteration 2, state space = (10,11,12) : (1,0,0) JPPD: Not update best state, Cost = 41463.286373 JPPD: Starting iteration 3, state space = (10,11,12) : (0,1,0) JPPD: Not update best state, Cost = 41463.286373 JPPD: Starting iteration 4, state space = (10,11,12) : (0,0,1) JPPD: Not update best state, Cost = 41463.286373
As you can see, the state space (10,11,12) : (0,0,0) is the one with the lowest cost. In other words, it would be cheapest not to do JPPD at all.
Also the subsequent log entry indicates that JPPD shouldn't be done:
JPPD: Will not use JPPD from query block SET$BADDE9FB_1 (#4)
But later on, strangely, the optimizer ignores that and, in the end, performs JPPD on all three query blocks:
JPPD: Checking validity of push-down in query block SET$BADDE9FB_1 (#4) JPPD: JPPD: Pushdown from query block SET$BADDE9FB_1 (#4) passed validity checks. JPPD: Performing join predicate push-down (final phase) from query block SET$BADDE9FB_1 (#4) to query block SEL$A0D20652 (#12) JPPD: Pushing predicate "BRIEFANREDE"."VIN01ID"(+)="VIN01"."ID"from query block SET$BADDE9FB_1 (#4) to query block SEL$A0D20652 (#12) JPPD: Push dest of pred 0xa8637960 is qb 0xbac44950:query block SEL$A0D20652 (#12)
JPPD: Performing join predicate push-down (final phase) from query block SET$BADDE9FB_1 (#4) to query block SEL$41289C4A (#11) JPPD: Pushing predicate "ORG12_RM_GESCHAEFTSFELD"."GES01_ID"(+)="GES01"."ID" from query block SET$BADDE9FB_1 (#4) to query block SEL$41289C4A (#11) JPPD: Push dest of pred 0xa8633870 is qb 0xcbbc3de8:query block SEL$41289C4A (#11)
JPPD: Performing join predicate push-down (final phase) from query block SET$BADDE9FB_1 (#4) to query block SEL$2DE89145 (#10) JPPD: Pushing predicate "EAM"."GES01_GESCHAEFTSBEZIEHUNG_GBZ"(+)="VIN01"."GES01_GBZ_GBZ" from query block SET$BADDE9FB_1 (#4) to query block SEL$2DE89145 (#10) JPPD: Push dest of pred 0x8fa13010 is qb 0x858777a8:query block SEL$2DE89145 (#10)
In the trace I haven't found any clue about what led the optimizer to suddenly start doing JPPD.
First and foremost, have I misinterpreted the trace?
If not, is anyone aware of any circumstances which could cause such behavior?
Best regards,
Nenad
https://nenadnoveljic.com/blog
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