Lesson Learned #521: Query Performance Regression with Multiple Execution Plans in Azure SQL
Some days ago, we were working on a service request where our customer asked why a query had degraded in performance. One possible issue could be that more than one execution plan is being used for a specific query. So I would like to share the steps we followed using QDS with DMVs. First, we executed this query to identify any queries that had more than one plan_id, which is often a sign that the optimizer has compiled multiple strategies to run the same query: SELECT q.query_id, qt.query_sql_text, q.query_hash, COUNT(DISTINCT p.plan_id) AS num_plans, STRING_AGG(CAST(p.plan_id AS VARCHAR), ', ') AS plan_ids FROM sys.query_store_query_text qt JOIN sys.query_store_query q ON qt.query_text_id = q.query_text_id JOIN sys.query_store_plan p ON q.query_id = p.query_id GROUP BY q.query_id, qt.query_sql_text, q.query_hash HAVING COUNT(DISTINCT p.plan_id) > 1 ORDER BY num_plans DESC; We got a list of queries and after some analysis, we found the one the customer was referring to. The query in question was a simple aggregate with a parameter: (@N int)SELECT count(Name),name FROM Notes where ID<@n group by Name As we found that they query has two plans, we executed the following TSQL to obtain the details of the executions. SELECT rs.execution_type_desc, rs.avg_duration / 1000 AS avg_duration_ms, rs.avg_cpu_time / 1000 AS avg_cpu_ms, rs.last_duration / 1000 AS last_duration_ms, rs.count_executions, rs.first_execution_time, rs.last_execution_time, p.plan_id, p.is_forced_plan, TRY_CONVERT(XML, p.query_plan) AS execution_plan_xml FROM sys.query_store_runtime_stats rs JOIN sys.query_store_plan p ON rs.plan_id = p.plan_id WHERE p.query_id = 2 ORDER BY rs.last_execution_time DESC; We got the following results: We could see the execution plan number 2 was executed less time but taking more time in average. Checking the execution plan XML we were able to identify an automatic update statistics was executed causing a new execution plan. Trying to give insights about possible causes, we wrote the following TSQL giving us when the statistics were updated directly from the execution plan XML. ;WITH XMLNAMESPACES (DEFAULT 'http://schemas.microsoft.com/sqlserver/2004/07/showplan') SELECT p.plan_id, stat.value('@Statistics', 'VARCHAR(200)') AS stats_name, stat.value('@LastUpdate', 'DATETIME') AS stats_last_updated, stat.value('@SamplingPercent', 'FLOAT') AS stats_sampling_percent FROM sys.query_store_plan AS p CROSS APPLY ( SELECT CAST(p.query_plan AS XML) AS xml_plan ) AS x OUTER APPLY x.xml_plan.nodes(' /ShowPlanXML/BatchSequence/Batch/Statements/StmtSimple/QueryPlan/OptimizerStatsUsage/StatisticsInfo' ) AS t(stat) WHERE p.query_id = 2; Well, we found another way to query directly the execution plan and include other information from Query Data Store. Enjoy!
