Plan değişimi P458

(@now datetime)WITH p AS (SELECT PlanHash, PlanXml, ROW_NUMBER() OVER (PARTITION BY PlanHash ORDER BY plan_id) AS rn FROM #px)
        INSERT perf.QdsPlanXml (PlanHash, PlanXmlCompressed, FirstSeenUtc, LastSeenUtc)
        SELECT PlanHash, COMPRESS(PlanXml), @now, @now FROM p
        WHERE rn = 1 AND NOT EXISTS (SELECT 1 FROM perf.QdsPlanXml AS e WHERE e.PlanHash = p.PlanHash)

Plan change case #P458 | DB: DB-12 | Improvement | Confidence: Low (review pending)
Query: (@now datetime)WITH p AS (SELECT PlanHash, PlanXml, ROW_NUMBER() OVER (PARTITION BY PlanHash ORDER BY plan_id) AS rn FROM #px) INSERT perf.QdsPlanXml (PlanHash, PlanXmlCompressed, FirstSeenUtc, LastSeenUtc) SELECT PlanHash, COMPRESS(PlanXml), @now, @now FROM p WHERE rn = ? AND NOT EXISTS (SELECT ? FROM perf.QdsPlanXml AS e WHERE e.PlanHash = p.PlanHash)
Change: 2026-10-05 11:00 UTC, plan 8465 → 8476, pattern: Alternating (10 switches)
Impact (Query Store, per execution): duration 106 → 5 ms (-95%); CPU 78.1 → 3.3 ms (-96%); reads 2,920 → 103 (-96%); rows 96.89 → 1.8 (-98%)
Plan difference: no operator-level difference
Candidate cause: STATS — Statistic perf.QdsPlanXml.PK_QdsPlanXml was updated 2026-10-05 01:11 (sampled 25.3%); 2 updates in the window; the plans use this statistic.
Candidate cause: WORKLOAD — The plans see a different workload: rows per execution 96.89 → 1.8. The plans may have been compiled for different parameters or batch sizes.
Candidate cause: PARAM — The two plans alternate in the same period (10 switches): plan choice depends on the parameter value rather than an event (parameter sensitivity).
Counter-evidence: WORK_PROPORTIONAL — Rows per execution changed 0.02× and duration 0.05×; duration per row 1.0943 → 2.7669 ms: the duration difference comes from the amount of data processed, not the plan.
Counter-evidence: HIGH_VARIANCE — The old plan's duration standard deviation is 349% of the average: the average is not reliable.
Counter-evidence: HIGH_VARIANCE — The new plan's duration standard deviation is 301% of the average: the average is not reliable.
Counter-evidence: SAME_SHAPE — The two plans have the same operator tree (operators, tables, indexes): the plan did not change; the difference is only in the compiled parameter values and row estimates. The slowness may come from the amount of data this parameter value processes or from waits, not from plan choice.
Confidence rationale: Low: strong counter-evidence (WORK_PROPORTIONAL, SAME_SHAPE).
Note: this is a candidate cause, not proof of the root cause.