Oracle plan history and plan diff

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Oracle Plan History records plan observations for sampled Oracle SQL so that Mini DBA can show when a statement's execution plan changed. Use it when a query became slower, started reading more data, or changed access path after statistics, object, bind, or schema changes.

Where to open it

Open Oracle Query Analysis, select a live or historical SQL row, and open the SQL detail modal. The Plan History tab lists the plan observations Mini DBA has captured for that SQL. If a debug plan-change workload was used, search Query History for the debug object name shown in the workload status.

What the tab shows

Plan History shows the captured time, plan hash, Oracle PLAN_HASH_VALUE where available, child cursor context, and plan text. When two observations differ, Mini DBA builds a visual diff so you can see which lines or operators changed.

Mini DBA first tries to use DBMS_XPLAN.DISPLAY_CURSOR for familiar Oracle plan text. If that is not available but GV$SQL_PLAN rows are visible, Mini DBA formats a readable fallback plan from the structured plan rows. This fallback is less rich than DBMS_XPLAN output but still preserves operation, object, cost, rows, bytes, and predicate evidence.

How to interpret a plan change

A changed plan is important when it aligns with a workload symptom. Look for:

Access path changes, such as table scan to index scan or the reverse.
Join method changes, such as nested loops, hash joins, or merge joins.
Changed object names or indexes.
Changed estimated rows or cost.
New sorts, aggregations, or temp-heavy operations.
Predicate changes that may indicate bind, implicit conversion, or statistics effects.

Plan change evidence should be reviewed with elapsed time, CPU, reads, rows processed, wait events, object statistics, and the timing of deployments or maintenance.

Nearby changes

The SQL detail modal can also show nearby schema-change events. This helps connect a plan change to an object change, new index, dropped index, altered column, or statistics-related event recorded by Mini DBA. Nearby changes are not proof of cause, but they shorten the investigation.