optimize query statements summary table by avoid decode plan by conditional pushdown

[crazycs520]

Background

PR #25843 enlarge the tidb_stmt_summary_max_stmt_count default value to 3000. It's great for diagnose problem since the statements_summary can record more SQL. But it will also slow down the performance when query statements_summary. Mostly the reasons affecting performance is cause by DecodePlan for each row in statements_summary.

#25892 already has optimized the performance by prune column. But it doesn't optimize for the following SQL which will be used in tidb-dashboard:

select * 
from `STATEMENTS_SUMMARY` 
where 
    digest='8019af26debae8aa7642c501dbc43212417b3fb14e6aec779f709976b7e521be' 
    and 
    unix_timestamp(summary_begin_time) = 1626769800;

The upper SQL need query all columns value, so #25892 which use prune column method can't optimize it.

But we can use conditional pushdown to do optimization. As you can see the condition is digest='8019af26debae8aa7642c501dbc43212417b3fb14e6aec779f709976b7e521be' and unix_timestamp(summary_begin_time) = 1626769800. Mostly There is very little data that meets this condition, so we can use conditional pushdown to only fetch the row which meets the condition.

Reference

How to do conditional pushdown for query system table?

• You can use MemTablePredicateExtractor to implements conditional pushdown.

You can reference the following PR which also do conditional pushdown for query system table:

• planner,executor,infoschema: add system tables tiflash_tables and tiflash_segments #18092
• executor: make SLOW_QUERY support query slow log at any time #14840

mentor

• crazycs520

If you have any questions about this issue, you can comment on this issue, I will reply you as soon as possible.

[TszKitLo40]

Can I try this issue? @crazycs520

[TszKitLo40]

/assign