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DBAs can use this column to provide information about why a cost estimate row stores a particular value.
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Then execute FLUSH OPTIMIZER_COSTS to tell the server to re-read the cost tables. To revert to the default, set the value to NULL. To override a default cost estimate (for an entry that specifies NULL), set the cost to a non- NULL value. If the server finds that the cost value is invalid (nonpositive) when it reads this table, it writes a warning to the error log. DBAs can change a cost estimate by updating this column. Otherwise, it uses the default estimate (the compiled-in value). If the value is non- NULL, the server uses it as the cost. If the server does not recognize the cost name when it reads this table, it writes a warning to the error log.
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The name of a cost estimate used in the cost model. The server_cost table contains these columns: Server_cost: Optimizer cost estimates for general server operationsĮngine_cost: Optimizer cost estimates for operations specific to particular storage engines The optimizer cost model database consists of two tables in the mysql system database that contain cost estimate information for operations that occur during query execution: 成本模型数据库 The server does not replicate cost table changes to replicas. Cost tables are specific to a given server instance.In particular, if the server re-reads the cost tables, any changed estimates apply only to subsequently started sessions. The in-memory cost estimates that are current when a client session begins apply throughout that session until it ends.The optimizer uses the in-memory cost values, so changes to the tables should be followed by FLUSH OPTIMIZER_COSTS to take effect. It is also easy to revert to a default by setting an entry's cost to NULL. Cost tables enable server administrators to easily adjust cost estimates by changing entries in the tables.This occurs when a storage engine is dynamically loaded or when a FLUSH OPTIMIZER_COSTS statement is executed. At runtime, the server may re-read the cost tables.Any NULL estimate indicates to the optimizer to use the compiled-in default. Any non- NULL cost estimate specified in the tables takes precedence over the corresponding compiled-in default cost constant. The server reads the cost model tables into memory at startup and uses the in-memory values at runtime.The configurable optimizer cost model works like this: The intent of these tables is to make it possible to easily adjust the cost estimates that the optimizer uses when it attempts to arrive at query execution plans. These estimates are stored in the server_cost and engine_cost tables in the mysql system database and are configurable at any time. The optimizer also has a database of cost estimates to use during execution plan construction. The optimizer has a set of compiled-in default “cost constants” available to it to make decisions regarding execution plans.
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To generate execution plans, the optimizer uses a cost model that is based on estimates of the cost of various operations that occur during query execution.