One case of SQL performance degradation caused by modifying implicit parameters

A customer found one that had been implemented for a long time （15.8 Hours ） Not yet completed SQL, and SQL Used 8 Two parallel processes ：

SQL The code is as follows ：

SELECT /*+ PARALLEL(a,8)*/ count(: "SYS_B_0")

FROM so1.ins_srv_attr_780 a

WHERE a.offer_inst_id IN

(

SELECT /*+parallel(b,8)*/ b.offer_inst_id

FROM so1.ins_offer_780 b

WHERE b.offer_type = : "SYS_B_1"

) AND

a.prod_srv_relat_id NOT IN

(

SELECT c.prod_srv_relat_id

FROM so1.ins_prod_ins_srv_780 c

);

SQL Contains a IN And a NOT IN, In the execution plan not in Of filter Inefficient steps , And this step also omits parallel hint（ Not the main problem ）. Why does the optimizer choose such an inefficient execution plan ？

The customer modified several implicit parameters ：

OPT_PARAM('_optimizer_use_feedback' 'false')

OPT_PARAM('_optimizer_join_factorization' 'false')

OPT_PARAM('_optimizer_enable_density_improvements' 'false')

OPT_PARAM('_optimizer_adaptive_cursor_sharing' 'false')

OPT_PARAM('_optimizer_extended_cursor_sharing_rel' 'none')

OPT_PARAM('_bloom_pruning_enabled' 'false')

OPT_PARAM('_gby_hash_aggregation_enabled' 'false')

OPT_PARAM('_optimizer_join_elimination_enabled' 'false')

OPT_PARAM('_optimizer_extended_cursor_sharing' 'none')

OPT_PARAM('_bloom_filter_enabled' 'false')

OPT_PARAM('_optimizer_squ_bottomup' 'false')

After performing troubleshooting , Find the culprit _optimizer_squ_bottomup：

Let's see if this parameter is the default TRUE When SQL Implementation of （ Incidentally, the parallel writing problem has been corrected ）：

SELECT /*+ PARALLEL(8) OPT_PARAM('_optimizer_squ_bottomup' 'true') */ count(1)

FROM so1.ins_srv_attr_780 a

WHERE a.offer_inst_id IN

(

SELECT b.offer_inst_id

FROM so1.ins_offer_780 b

WHERE b.offer_type = 'OFFER_PLAN_BBOSS'

) AND a.prod_srv_relat_id NOT IN

(

SELECT c.prod_srv_relat_id

FROM so1.ins_prod_ins_srv_780 c

);

See the following figure directly for the effect ：

system parameter , In particular, implicit parameters , I haven't met bug Under the circumstances , Generally, modification is not recommended , This case is a very obvious lesson .