22.214.171.124 PDB (FROM 126.96.36.199) USES UNWANTED/SLOW PARALLEL QUERY EVEN WHEN DISABLED ON SESSION LEVEL
(Doc ID 2072729.1)
Last updated on MARCH 10, 2020
Applies to:Oracle Database - Enterprise Edition - Version 188.8.131.52 and later
Oracle Database Cloud Schema Service - Version N/A and later
Oracle Database Exadata Cloud Machine - Version N/A and later
Oracle Database Exadata Express Cloud Service - Version N/A and later
Oracle Cloud Infrastructure - Database Service - Version N/A and later
Information in this document applies to any platform.
upgraded an application from db 184.108.40.206 to 220.127.116.11. Now we notice very strange and highly unwanted behavior that strongly delays/slows queries that were previously executing fast. Investigation has finally shown that dictionary queries keep on executing parallel (SQL monitoring show PX plans) despite disabling parallel query/dml/ddl on session level and despite adding no_parallel hints to the query. The only thing that prevents parallel execution is setting parallel_max_servers to zero.
This only occurs with upgraded and next plugged in databases. With a fresh PDB (created in 12c, so no 11g history) from PDB$SEED the disable parallel query does work.
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