mirror of https://github.com/citusdata/citus.git
889aa92ac0
DESCRIPTION: Fixed a bug in EXPLAIN ANALYZE to prevent unintended (duplicate) execution of the (sub)plans during the explain phase. Fixes #4212 ### 🐞 Bug #4212 : Redundant (Subplan) Execution in `EXPLAIN ANALYZE` codepath #### 🔍 Background In the standard PostgreSQL execution path, `ExplainOnePlan()` is responsible for two distinct operations depending on whether `EXPLAIN ANALYZE` is requested: 1. **Execute the plan** ```c if (es->analyze) ExecutorRun(queryDesc, direction, 0L, true); ``` 2. **Print the plan tree** ```c ExplainPrintPlan(es, queryDesc); ``` When printing the plan, the executor should **not run the plan again**. Execution is only expected to happen once—at the top level when `es->analyze = true`. --- #### ⚠️ Issue in Citus In the Citus implementation of `CustomScanMethods.ExplainCustomScan = CitusExplainScan`, which is a custom scan explain callback function used to print explain information of a Citus plan incorrectly performs **redundant execution** inside the explain path of `ExplainPrintPlan()` ```c ExplainOnePlan() ExplainPrintPlan() ExplainNode() CitusExplainScan() if (distributedPlan->subPlanList != NIL) { ExplainSubPlans(distributedPlan, es); { PlannedStmt *plan = subPlan->plan; ExplainOnePlan(plan, ...); // ⚠️ May re-execute subplan if es->analyze is true } } ``` This causes the subplans to be **executed again**, even though they have already been executed during the top-level plan execution. This behavior violates the expectation in PostgreSQL where `EXPLAIN ANALYZE` should **execute each node exactly once** for analysis. --- #### ✅ Fix (proposed) Save the output of Subplans during `ExecuteSubPlans()`, and later use it in `ExplainSubPlans()` |
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