pg_stat_statements: Add rows_filtered column #6
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NikolayS
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Add a new rows_filtered column to pg_stat_statements that tracks the total number of rows removed by filter conditions across all plan nodes. This helps identify queries that would benefit from better indexing. The feature is controlled by a new GUC pg_stat_statements.track_rows_filtered (default: off). When enabled, the module uses INSTRUMENT_ROWS instrumentation to collect nfiltered1 (scan/join qual) and nfiltered2 (recheck conditions) from all plan nodes via planstate_tree_walker. Key implementation details: - Uses INSTRUMENT_ROWS (not INSTRUMENT_ALL) to avoid timing overhead - Only increments tuple counters, which has negligible performance impact - GUC is superuser-only (PGC_SUSET), can be changed at runtime - Stats file header bumped to invalidate old stats files This addresses a common DBA need to identify inefficient sequential scans where adding an index could significantly improve performance.
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Add a
rows_filteredcolumn topg_stat_statementsto track the total number of rows removed by filter conditions during query execution. This metric helps identify queries that may benefit from better indexing, as highrows_filteredvalues indicate that many rows are being scanned but then discarded.The implementation:
INSTRUMENT_ROWSbeforeExecutorStart(usingINSTRUMENT_ROWSinstead ofINSTRUMENT_ALLto avoid the overhead of timing instrumentation)ExecutorEndto sumnfiltered1(scanqual/joinqual) andnfiltered2(other quals) from all nodesExample output:
This helps identify inefficient queries where many rows are scanned but few are returned, suggesting an index might help.