*: optimize query statements summary table by avoid decode plan for each row

[crazycs520]

What problem does this PR solve?

Before this PR, query the statemens summary table maybe very slow cause by decode plan, event the query doesn't need to read the plan.

Bench

Generate 3000 statements to filling statemens summary , and each SQL length is 10240 byte.

1. Use loadgen to generated load:

   bin/loadgen payload  gen-stmt --stmt-cnt 3000 --query-len 10240 --host 172.16.5.40 --port 4019  --concurrency 10

   The generated SQL is look like this:

   select * from t_gen_stmt_1 where a = 1 or b = 1 or c = 1 
   union all
   select * from t_gen_stmt_2 where a = 2 or b = 2 or c = 2 
   union all
   ...
   ...
   ...

2. Wait generated statements(SQL) fill in the statemens summary.

3. Bench the SQL: select count(*) from information_schema.cluster_statements_summary_history;

Bellow is the bench result compare with the master:

	Master	This PR
Query duration	3.4s	7ms
Query memory consume	500MB	< 1MB

Related TiDB memory consume metric is below:

Master:

This PR:

# What is changed and how it works?

This PR add a special stmtSummaryReader to read the statemens summary table data, and only 'decode' the needed column, avoid decode plan for each table row.

Also, this PR implements prune columns for LogicalMemTable if the mem table is statemens summary table. Otherwise, stmtSummaryReader still has to read all column since LogicalMemTable doesn't prune column.

Related changes

• N/A

Check List

Tests

• Unit test
• Manual test

Side effects

• • N/A

Release note

• N/A

[sre-bot]

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[crazycs520]

@djshow832 @winoros @ClSlaid PTAL

[crazycs520]

/run-all-tests

[ClSlaid]

/LGTM

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[ClSlaid]

LGTM

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[crazycs520]

/run-all-tests

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This pull request has been approved by:

• AilinKid
• djshow832

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[crazycs520]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 4cf9788