12.1.0.2 PDB (FROM 11.2.0.4) USES UNWANTED/SLOW PARALLEL QUERY EVEN WHEN DISABLED ON SESSION LEVEL

(Doc ID 2072729.1)

Last updated on JANUARY 31, 2018

Applies to:

Oracle Database - Enterprise Edition - Version 12.1.0.2 and later
Information in this document applies to any platform.

Symptoms

upgraded an application from db 11.2.0.4 to 12.1.0.2. 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.

Cause

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