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Execution Plan Cache for Optimizing UK/PK Queries

Last updated: 2022-03-07 10:27:50

    Feature Overview

    In MySQL, SQL statement execution is divided into four stages: parsing, preparation, optimization, and execution. The execution plan cache feature is only available for prepared statements. After the feature is enabled, the first three stages will be skipped when executing a prepared statement, greatly boosting query performance.

    In MySQL 8.0 20210830, the execution plan cache takes effect only for queries using unique keys (UKs) or primary keys (PKs). We will cover more types of queries in later versions.

    Supported Versions

    Kernel version: MySQL 8.0 20210830 and above.

    Use Cases

    This feature is mainly used to improve the query performance when executing short prepared statements with UKs or PKs on TencentDB instances. However, the extent to which performance may improved depends on your business.

    Impact on Performance

    • For UK and PK SQL statements, the delay is reduced by 20%-30% and the throughput is improved by 20%-30% after the execution plan cache is enabled (according to the sysbench test using the point_select.lua script).
    • Memory usage will increase after the execution plan cache is enabled.

    Use Instructions

    You can use the cdb_plan_cache parameter to enable or disable the execution plan cache and the cdb_plan_cache_stats parameter to query information about cache hits. Note that only accounts with the tencentroot permission can use the two parameters.

    Parameter Effective Immediately Type Default Value Valid Values/Value Range Description
    cdb_plan_cache Yes bool false true/false Enable or disable the execution plan cache. Only accounts with the tencentroot permission can use the parameter.

    You can run the show cdb_plan_cache command to query information about execution plan cache hits. The command will return the following fields:

    Field Description
    sql An SQL statement with the question mark (?) which represents that the execution plan of this statement has been cached.
    mode SQL cache mode. Currently, only the prepare mode is supported.
    hit The number of hits in this thread

    After cdb_plan_cache_stats is enabled, cache hit information will be recorded, affecting database performance.


    Currently, you cannot directly modify the values of the above parameters. If needed, submit a ticket for assistance.

    SQL Execution Status

    You can run show profile to show the status at each stage of SQL statement execution. But when the execution plan cache is hit, the status of optimizing, statistics, and preparing will be omitted.

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