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Query plan monitoring is the act of monitoring the performance and output of a query during execution. A query plan provides clear, logical steps to perform tasks such as querying databases and retrieving data. Just like creating a query plan is a complex task, monitoring it for performance and output is a complex process. Monitoring is a multidimensional task and the plan needs to monitor multiple aspects of a query such as performance, nested steps and their performance, and speed of retrieval.
Query plan monitoring is also known as SQL plan monitoring.
A query plan is a complex set of steps framed depending on what the goal of the query is. For example, a query retrieving the names of all employees in the finance department of a small company would likely be a simpler query than one that retrieves the names and passport numbers of employees whose names contain certain letters and who reside in certain states. Complex plans can have nested queries, loops or branches and each complexity may mean querying a different set of databases, data sources or tables in different locations.
While it is simple to monitor query plans on simple queries, it takes a detailed review of query plans on complex queries. For example, complex queries with much nesting and branching can be expensive and consume a lot of time and system resources. It is the job of the query plan to identify the specific query part which has been causing a drop in performance, and that can be a time-consuming task.