Optimizer Chooses Expensive Index Full Scan over Index Fast Full Scan or Full Table Scan from 12.1 (Doc ID 2382922.1)

Last updated on MARCH 18, 2022

Applies to:

Symptoms

• Optimizer chooses expensive index full scan over index fast full scan or full table scan which are very faster.
• Queries take hours to complete using index full scan while it runs in seconds using index fast full scan.
• Problem happens after upgrading the database from lower version to 12.1.0.2 or higher.
• Normal 'SELECT COUNT(column) FROM table' queries are using index full scan over index fast full scan or full table scan.
• Problem happens even after the fix for <Bug 17863980> is included in 12.1.0.2.
• Good plan runs with index fast full scan or full table scan using parallelism.
• Bad plan runs with index full scan in serial mode without using parallelism.
• Cost of the plan using index fast full scan and parallelism is lesser than index full scan in serial. However, optimizer chooses high cost inefficient plan.
• Setting the parameter "_optimizer_cost_model"=IO gives the desired IFFS/FTS plan.
• Setting OPTIMIZER_FEATURES_ENABLE to 11.2 or 10.2 do not help.
• Deleting the system statistics gives desired plan.
• Setting higher SREADTIM system statistics gives desired plan but that may impact other SQL statements.
• Setting DB_FILE_MULTIBLOCK_READ_COUNT to non-default value causing the bad plans.

Changes

Upgrading the Database to 12.1 or higher.
Changes to hardware like disk storage etc.

Cause

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