Mysql optimizer statistics how to#

The preceding program units observe the GLOBAL_TEMP_TABLE_STATS statistics preference. GET_ obj_STATS, where obj is TABLE, COLUMN, or INDEX SET_ obj_STATS, where obj is TABLE, COLUMN, or INDEX The following procedures do not commit for transaction-specific temporary tables, so that rows in these tables are not deleted:ДELETE_ obj_STATS, where obj is TABLE, COLUMN, or INDEX Table 10-2 Optimizer Statistics for Global Temporary Tables Characteristic Implicitly created when optimizer considers it usefulД®ffect of Creation on Existing TransactionsДROP PRIVATE TEMPORARY TABLE, or implicitly dropped at end of session ( PRESERVE DEFINITION) or transaction ( DROP DEFINITION) Until table is explicitly dropped, or end of session ( PRESERVE DEFINITION) or transaction ( DROP DEFINITION)Д¬REATE GLOBAL TEMPORARY TABLE (supports AS SELECT)Д¬REATE PRIVATE TEMPORARY TABLE (supports AS SELECT) Session that created table (in USER_PRIVATE_TEMP_TABLES view, which is based on a V$ view) Memory or tempfiles, but only for the duration of the session or transaction

Table 10-1 Important Characteristics of Temporary Tables Characteristic Note that the database allocates storage space when a session first inserts data into a global temporary table, not at table creation. The tables differ in where they store data, how they are created and dropped, and in the duration and visibility of metadata. Unlike global and private temporary tables, DBMS_STATS cannot gather statistics for cursor-duration temporary tables. The ON COMMIT clause of CREATE PRIVATE TEMPORARY TABLE indicates whether the table is transaction-specific ( DROP DEFINITION) or session-specific ( PRESERVE DEFINITION).ДЄ cursor-duration temporary table is an implicitly created memory-only object that is associated with a cursor. The table is private because the definition is visible only to the session that created the table. Optimizer statistics for global temporary tables can be shared or session-specific.ДЄ private temporary table is an explicitly created object, defined by private memory-only metadata, that stores intermediate session-private data for a specific duration. The ON COMMIT clause of CREATE GLOBAL TEMPORARY TABLE indicates whether the table is transaction-specific ( DELETE ROWS) or session-specific ( PRESERVE ROWS). The table is global because the definition is visible to all sessions. The tables differ as follows:ДЄ global temporary table is an explicitly created persistent object that stores intermediate session-private data for a specific duration.

In all types of temporary table, the data is only visible to the session that inserts it. Temporary tables are classified as global, private, or cursor-duration. You can access these statistics using data dictionary views. Number of distinct values (NDV) in a columnДЄs shown in Figure 10-1, the database stores optimizer statistics for tables, columns, indexes, and the system in the data dictionary. Optimizer statistics include the following: The optimizer estimates the cost for the access, determines the cost for possible plans, and then picks the execution plan with the lowest cost. The optimizer uses statistics to get an estimate of the number of rows (and number of bytes) retrieved from a table, partition, or index. The better the statistics and the more precisely it describes the actual data, the faster the plan will be, because the optimizer image of reality will be closer to the actual reality.ДЄnd, of course, when not to use indexes, when up-to-date statistics is enough.The optimizer cost model relies on statistics collected about the objects involved in a query, and the database and host where the query runs. To make this decision optimizer can examine the actual table data, but with multi gigabyte and terabyte tables, the only practical solution is to use various data statistics that were collected in advance.

Mysql optimizer statistics how to#

When your SQL query reaches the DBMS, it's the optimizer's job to decide how to execute it for you to get the result as fast as possible. In this talk you'll learn what data statistics MariaDB and MySQL can collect, what statements do that, how to tell the optimizer to use it (it won't necessarily do it automatically!) and how it can make your queries many times faster.