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Statement Outline
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Last updated: 2025-11-28 10:54:18
Statement Outline
Last updated: 2025-11-28 10:54:18
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Overview

SQL tuning is a crucial step in improving database performance. To avoid the impact when the optimizer fails to select an appropriate execution plan, TXSQL provides the outline feature for you to bind execution plans. TDSQL-C for MySQL allows you to use hints to manually bind execution plans. The hint information contains the optimization rule for SQL statements, algorithm to be used, and index for data scan. An outline relies on hints to specify execution plans. ‌TDSQL-C for MySQL provides the mysql.outline system table for you to add plan binding rules and the cdb_opt_outline_enabled switch for you to enable/disable the outline feature.

Supported Versions

‌TDSQL-C for MySQL 8.0 (kernel version 3.1.10) or later.

Use Cases

This feature is suitable for scenarios where an execution plan in the production environment has poor performance (for example, the index in the execution plan is incorrect), but you don't want to modify SQL statements and release a new version to fix this problem.

Use Limits

The outline syntax uses a new syntax form:
Configure outline information: outline "sql" set outline_info "outline";
Clear outline information: outline reset ""; outline reset all;
Refresh outline information: outline flush;
Below are the outline use methods with the following schemas as examples:
create table t1(a int, b int, c int, primary key(a));
create table t2(a int, b int, c int, unique key idx2(a));
create table t3(a int, b int, c int, unique key idx3(a));
Parameter
Effective Immediately
Type
Default Value
Valid Values
Description
cdb_opt_outline_enabled
yes
bool
false
true/false
Whether to enable the outline feature.
Note:
Currently, you cannot directly modify the values of the above parameter. If needed, submit a ticket for assistance.

Binding an outline

To bind an outline directly, you can replace one SQL statement with another, without changing the syntax of the SQL. This simply adds some HINT information to instruct the optimizer on how to execute the statement. The syntax is in the format of outline "sql" set outline_info "outline";. Note that the string after outline_info must start with "OUTLINE:", which is followed by the SQL statement with the hint information added. For example, you can add the index in column a to table t2 in the SQL statement select * from t1, t2 where t1.a = t2.a as follows:
outline "select * from t1, t2 where t1.a = t2.a" set outline_info "OUTLINE:select * from t1, t2 use index(idx2) where t1.a = t2.a";

Binding optimizer hint

To make the feature more flexible, TXSQL allows you to add optimizer hints incrementally to SQL statements. You can also implement the same feature by directly binding an outline. The syntax is in the format of outline "sql" set outline_info "outline";. Note that the string after outline_info must start with "OPT:", which is followed by the optimizer hint information to be added. For example, you can specify SEMIJOIN of MATERIALIZATION/DUPSWEEDOUT for the SQL statement select *from t1 where t1.a in (select b from t2) as follows:
outline "select * from t1 where t1.a in (select b from t2)" set outline_info "OPT:2#qb_name(qb2)";
outline "select * from t1 where t1.a in (select b from t2)" set outline_info "OPT:1#SEMIJOIN(@qb2 MATERIALIZATION, DUPSWEEDOUT)";
You can add only one optimizer hint to the original SQL statement at a time and must comply with the following rules:
The OPT keyword must follow ".
':' must be placed before the new statement to be bound.
You must add two fields (query block number#optimizer hint string), which must be separated with "#" (e.g., "OPT:1#max_execution_time(1000)").

Binding index hint

To make the feature more flexible, TXSQL allows you to add index hints incrementally to SQL statements. You can also implement the same feature by directly binding an outline. The syntax is in the format of outline "sql" set outline_info "outline";. Note that the string after outline_info must start with "INDEX:", which is followed by the index hint information to be added. For example, you can add the index idx1 of USE INDEX in FOR JOIN type to the table t1 in the database test in query block 3 for the SQL statement select * from t1 where t1.a in (select t1.a from t1 where t1.b in (select t1.a from t1 left join t2 on t1.a = t2.a)) as follows:
outline "select * from t1 where t1.a in (select t1.a from t1 where t1.b in (select t1.a from t1 left join t2 on t1.a = t2.a))" set outline_info "INDEX:3#test#t1#idx1#1#0";
You can add only one index hint to the original SQL statement at a time and must comply with the following rules:
The INDEX keyword must follow ".
':' must be placed before the new statement to be bound.
You must add five fields (query block number#db_name#table_name#index_name#index_type#clause).
Here, index_type has three valid values (0: INDEX_HINT_IGNORE; 1: INDEX_HINT_USE; 2: INDEX_HINT_FORCE), and clause also has three valid values (1: FOR JOIN; 2: FOR ORDER BY; 3: FOR GROUP BY), which must be separated by "#" (e.g., "INDEX:2#test#t2#idx2#1#1", indicating to bind the index idx2 in USE INDEX FOR JOIN type to the table test.t2 in the second query block).

Deleting the outline information of a SQL statement

TXSQL allows you to delete the outline binding information from a SQL statement. The syntax is in the format of outline reset "sql";. For example, to delete the outline information from select * from t1, t2 where t1.a = t2.a, run the following statement: outline reset "select* from t1, t2 where t1.a = t2.a";.

Clearing all outline information

TXSQL allows you to clear all outline binding information in the kernel. The syntax is outline reset all, and the execution statement is outline reset all;.
There may be some specific problems in the production environment where you must bind an index. In this case, you can directly configure an outline for binding. You should analyze the possible performance compromise after configuring an outline and bind an outline only if the compromised performance is acceptable. You can consult kernel engineers if necessary.
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