Plan Regression
A performance deterioration caused when an unchanged query begins using a materially worse execution plan after changes in statistics, data distribution, bind values, optimizer behavior, or software version.
SQL performance can regress even when application code has not changed. Refreshed statistics, data growth or skew, a new index, optimizer-version changes, or different bind values may lead to another execution plan that performs poorly for the current distribution.
Diagnosis should not rely on elapsed time alone. Plan identity, estimated versus actual cardinality, buffer gets, physical I/O, and tail latency under concurrency help separate a real plan regression from workload growth. Oracle Database and PL/SQL: architecture, SQL, and performance provides the surrounding tuning context.