Other hints under the global hint type

Other hints under the global hint type allow developers and database administrators to perform fine-grained control over the execution of SQL statements. The following table lists the other types of hints supported:

APPEND Hint

The APPEND hint enables the direct load feature for the INSERT INTO SELECT statement. For more information, see Use the INSERT INTO SELECT statement to directly load data.

Syntax

/*+ APPEND */

Examples

INSERT /*+ append enable_parallel_dml parallel(16) */ INTO t2
SELECT * FROM t1;

CURSOR_SHARING_EXACT Hint

The CURSOR_SHARING_EXACT hint specifies that parameterization is disabled at the query level.

OceanBase Database allows you to replace literals in an SQL statement with bind variables. This feature is controlled by the CURSOR_SHARING variable. When cursor_sharing='exact', parameterization is not required. For more information, see cursor_sharing.

Syntax

/*+ CURSOR_SHARING_EXACT */

Examples

The following example shows a query that disables parameterization by using the CURSOR_SHARING_EXACT hint. After executing the query with two sets of parameters four times, two query plans are generated in the plan cache for different parameters.

alter system flush plan cache global;
SELECT /*+ CURSOR_SHARING_EXACT */ * FROM t1 WHERE c1=5;
SELECT /*+ CURSOR_SHARING_EXACT */ * FROM t1 WHERE c1=5;
SELECT /*+ CURSOR_SHARING_EXACT */ * FROM t1 WHERE c1=6;
SELECT /*+ CURSOR_SHARING_EXACT */ * FROM t1 WHERE c1=6;

SELECT sql_id, plan_id, statement FROM oceanbase.gv$ob_plan_cache_plan_stat where query_sql like "SELECT /*+ CURSOR_SHARING_EXACT */ * FROM t1 WHERE c1=%";
+----------------------------------+---------+---------------------------------------------------------+
| sql_id                           | plan_id | statement                                               |
+----------------------------------+---------+---------------------------------------------------------+
| E024EB33213BF501D4CA7ABB81A195B5 |   13249 | SELECT /*+ CURSOR_SHARING_EXACT */ * FROM t1 WHERE c1=5 |
| E024EB33213BF501D4CA7ABB81A195B5 |   13250 | SELECT /*+ CURSOR_SHARING_EXACT */ * FROM t1 WHERE c1=6 |
+----------------------------------+---------+---------------------------------------------------------+

DIRECT Hint

The DIRECT hint enables the direct load feature for the LOAD DATA and INSERT statements, improving the efficiency and performance of data import.

Syntax

The syntax of the DIRECT hint is as follows:

/*+ DIRECT (/*+ direct(need_sort, max_errors_allowed, load_mode) */)*/

Parameters

• need_sort: specifies whether to sort the imported data. Valid values: true and false. true indicates that the data needs to be sorted. false indicates that the data does not need to be sorted.

• max_errors_allowed: specifies the maximum number of error rows allowed. If the number of error rows exceeds this value, the import process will fail.

• load_mode: specifies the import mode. Valid values:

  • full: the default value, indicating a full import.
  • inc: indicating an incremental import, which supports the INSERT and IGNORE semantics.
  • inc_replace: indicating an incremental import that does not check for duplicate primary keys, equivalent to the REPLACE semantics of an incremental import.

Examples

DIRECT Hint in LOAD DATA Statements

Enable incremental direct load

LOAD DATA /*+ DIRECT(true, 0, inc) */
INFILE 'datafile.txt'
INTO TABLE mytable
FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"'
LINES TERMINATED BY '\n';

In this example, DIRECT(true, 0, inc) enables incremental direct load, sorts the data, and allows up to 0 error rows.

Enable full direct load

LOAD DATA
/*+ PARALLEL(4) DIRECT(true, 0, full) */
REMOTE_OSS INFILE 'oss://example.com/datafile.csv'
INTO TABLE my_table
FIELDS TERMINATED BY ',' ENCLOSED BY '"'
LINES TERMINATED BY '\n';

In this example, PARALLEL(4) specifies a parallelism of 4, and DIRECT(true, 0, full) enables full direct load, sorts the data, and allows up to 0 error rows.

For more information, see Direct load.

DIRECT Hint in INSERT INTO SELECT Statements

To enable direct load in INSERT INTO SELECT statements, you must use the enable_parallel_dml option in the format: /*+ direct(bool, int, load_mode)} enable_parallel_dml PARALLEL(N) */.

Enable incremental direct load

-- Enable incremental direct load and parallel DML to incrementally import data from old_table to new_table
INSERT /*+ direct(true, 0, 'inc') enable_parallel_dml PARALLEL(4) */ INTO new_table (id, name, value)
SELECT id, name, value
FROM old_table;

In this example, direct(true, 0, 'inc') enables incremental direct load, sorts the data, and allows up to 0 error rows; enable_parallel_dml enables parallel DML; and PARALLEL(4) sets the parallelism to 4.

Enable full direct load

-- Enable full direct load and parallel DML to import data from old_table to new_table
INSERT /*+ direct(true, 0, 'full') enable_parallel_dml PARALLEL(4) */ INTO new_table (id, name, value)
SELECT id, name, value
FROM old_table;

In this example, direct(true, 0, 'full') enables full direct load, sorts the data, and allows up to 0 error rows; enable_parallel_dml enables parallel DML; and PARALLEL(4) sets the parallelism to 4.

For more information, see Use the INSERT INTO SELECT statement to directly load data.

NO_DIRECT Hint

The NO_DIRECT hint disables the direct load feature in LOAD DATA, INSERT INTO SELECT, and CREATE TABLE AS SELECT statements.

Syntax

The syntax of the NO_DIRECT hint is as follows:

/*+ NO_DIRECT */

Parameters

NO_DIRECT: forces a single SQL statement to not use direct load. If an SQL statement contains this hint, it ignores other direct load hints and executes a regular import.

Examples

• Use the NO_DIRECT hint in a LOAD DATA statement

  LOAD DATA /*+ NO_DIRECT */ [REMOTE_OSS | LOCAL] INFILE 'file_name' INTO TABLE table_name [COMPRESSION]...
  

• Use the NO_DIRECT hint in an INSERT INTO SELECT statement

  INSERT /*+ NO_DIRECT */ INTO  table_name select_sentence
  

• Use the NO_DIRECT hint in a CREATE TABLE AS SELECT statement

  CREATE /*+ NO_DIRECT */ TABLE table_name [AS] select_sentence
  

DYNAMIC_SAMPLING Hint

The DYNAMIC_SAMPLING hint specifies whether to enable dynamic sampling for a query.

Syntax

/*+ DYNAMIC_SAMPLING ( 0 | 1 ) */

Parameter description

The parameters of the DYNAMIC_SAMPLING hint are described as follows:

• When the parameter is 0, dynamic sampling is disabled.
• When the parameter is 1, dynamic sampling is enabled.

Examples

The following example shows how to enable dynamic sampling by using the DYNAMIC_SAMPLING hint.

SELECT /*+ dynamic_sampling(1) */ *
FROM t1 WHERE c1 LIKE "%abc%" AND c2 LIKE "%abc%";

USE_PX Hint

The USE_PX hint forces the server to execute an SQL statement in parallel execution (PX) mode. PX mode allows an SQL statement to be executed in multiple threads, thereby improving query performance. Typically, the USE_PX hint is used with the PARALLEL hint to specify the number of threads for parallel execution. By default, the system uses the USE_PX hint.

Syntax

The syntax of the USE_PX hint is as follows:

/*+ USE_PX */

Examples

The following example shows how to use the USE_PX hint:

SELECT /*+ USE_PX PARALLEL(4)*/ e.dept_id, sum(e.salary)
FROM emp e
WHERE e.dept_id = 1001 GROUP BY e.dept_id;

ENABLE_PARALLEL_DML Hint

The ENABLE_PARALLEL_DML hint specifies to enable parallel DML for the current query. Its reverse hint is DISABLE_PARALLEL_DML, which is used to disable parallel DML.

For more information, see Parallel DML.

Considerations

• When you use /*+ENABLE_PARALLEL_DML PARALLEL(n)*/, the system first considers using distributed parallel DML (PDML). If the current environment does not support PDML, the system falls back to DAS concurrent write as a performance optimization.

• If the session-level parallelism is set to a fixed value, the execution behavior remains consistent with the above behavior.

• Even if the current environment does not support parallel DML (PDML), if the system parameter _enable_parallel_das_dml is set to true using the ALTER SYSTEM SET command (its default value is false), the system will not enable DAS (Distributed Active Storage) parallel write operations, even if the /*+ENABLE_PARALLEL_DML PARALLEL(n)*/ hint is used.

• If _enable_parallel_das_dml is set to true using the ALTER SYSTEM SET command (default is false) and the session-level parallelism is already set to a fixed value, then even if the current environment does not support PDML, using the /*+ENABLE_PARALLEL_DML PARALLEL(n)*/ hint will not enable DAS parallel write.

Syntax

/*+ ENABLE_PARALLEL_DML */

Examples

insert /*+ enable_parallel_dml parallel(8) */ into t2 select * from t1;

DISABLE_PARALLEL_DML Hint

The DISABLE_PARALLEL_DML hint specifies to disable parallel DML for the current query. Its reverse hint is ENABLE_PARALLEL_DML, which is used to enable parallel DML.

For more information, see Parallel DML.

Syntax

/*+ DISABLE_PARALLEL_DML */

Examples

insert /*+ disable_parallel_dml parallel(8) */ into t2 select * from t1;

ENABLE_PARALLEL_DAS_DML Hint

The ENABLE_PARALLEL_DAS_DML hint specifies to enable the use of Distributed Data Access Service (DAS) concurrent write optimization for the current DML statement. Its reverse hint is DISABLE_PARALLEL_DAS_DML.

Considerations

• The ENABLE_PARALLEL_DAS_DML hint must be used with the ENABLE_PARALLEL_DML hint.

• When the /*+ENABLE_PARALLEL_DAS_DML ENABLE_PARALLEL_DML PARALLEL(n)*/ hint is used, the system will treat DAS concurrent write as a forced option and execute operations based on the parallelism specified by PARALLEL(n).

• If the system parameter _enable_parallel_das_dml is set to true using the ALTER SYSTEM SET statement (its default value is false), the /*+ENABLE_PARALLEL_DAS_DML ENABLE_PARALLEL_DML PARALLEL(n)*/ hint becomes invalid. In this case, the system will not perform DAS parallel write operations.

Syntax

/*+ ENABLE_PARALLEL_DAS_DML */

Examples

insert /*+ ENABLE_PARALLEL_DAS_DML ENABLE_PARALLEL_DML PARALLEL(10)*/
into t1 select * from t2;

DISABLE_PARALLEL_DAS_DML Hint

The DISABLE_PARALLEL_DAS_DML hint specifies to disable the use of Distributed Data Access Service (DAS) concurrent write optimization for the current DML statement. Its reverse hint is ENABLE_PARALLEL_DAS_DML.

Considerations

• The DISABLE_PARALLEL_DAS_DML hint must be used with the ENABLE_PARALLEL_DML hint.

• When the /*+DISABLE_PARALLEL_DAS_DML ENABLE_PARALLEL_DML PARALLEL(n)*/ hint is used, the system will disable DAS concurrent write, even if other concurrent write optimization options such as PDML are enabled.

Syntax

/*+ DISABLE_PARALLEL_DAS_DML */

Examples

insert /*+ DISABLE_PARALLEL_DAS_DML ENABLE_PARALLEL_DML PARALLEL(10)*/
into t1 select * from t2;

LOAD_BATCH_SIZE Hint

The LOAD_BATCH_SIZE hint specifies the size of the batch of records to be inserted in each LOAD DATA statement.

In the LOAD_BATCH_SIZE hint, the batch_size parameter specifies the size of the batch of records to be inserted in each LOAD DATA statement. For more information about LOAD DATA, see LOAD DATA statement.

Syntax

/*+ LOAD_BATCH_SIZE ( batch_size ) */

Examples

-- Use four parallel processes to import data, appending new data to the end of the table, and set each batch to process 1000 records using the `LOAD_BATCH_SIZE` hint.
LOAD DATA /*+ PARALLEL(4) APPEND LOAD_BATCH_SIZE(1000) */
INFILE '/home/admin/test.csv' INTO TABLE t1;

LOG_LEVEL Hint

The LOG_LEVEL hint specifies the log level for the current query.

In the LOG_LEVEL hint, the log_level parameter specifies the log level. Common log levels include ERROR, WARN, INFO, TRACE, and DEBUG.

For more information about log levels, see Log levels.

Syntax

/*+ LOG_LEVEL ( [']log_level['] ) */

Examples

-- Use the `LOG_LEVEL` hint to specify the `TRACE` log level.
SELECT /*+ LOG_LEVEL(TRACE) */ *
FROM employees e
WHERE e.department_id = 1001;

MAX_CONCURRENT Hint

The MAX_CONCURRENT hint specifies the maximum number of concurrent queries allowed.

In the MAX_CONCURRENT hint, the intnum parameter specifies the maximum number of concurrent queries allowed. If the number of concurrent queries exceeds the allowed maximum, an error is returned when the query is executed. If intnum is set to 0, an error is always returned when the query is executed.

Note that currently, the MAX_CONCURRENT hint cannot be directly used in queries. Instead, you can create an outline containing only the MAX_CONCURRENT hint to limit the concurrency of specific SQL statements with a given SQL_ID.

Syntax

/*+ MAX_CONCURRENT ( intnum ) */

Examples

-- This query creates an outline named otl1 associated with the identifier 'EC102CB006383D732BC98797601D9B3B' and specifies a maximum of 10 concurrent executions for the corresponding query.
CREATE OUTLINE otl1 ON 'EC102CB006383D732BC98797601D9B3B'
USING HINT /*+ max_concurrent(10) */;

MONITOR Hint

The MONITOR hint enables capturing the execution process of the query in the SQL Plan Monitor. You can use the MONITOR hint to record the execution process of the query in the SQL Plan Monitor.

For queries that do not use parallel execution, OceanBase Database does not record their execution process in the SQL Plan Monitor.

For more information about the SQL Plan Monitor, see the Real-time SQL Plan Monitor section in Display real-time execution plans.

Syntax

/*+ MONITOR */

Examples

-- Use the `MONITOR` hint to enable the SQL Plan Monitor.
SELECT /*+monitor*/ c1, SUM(distinct c2) FROM t1 GROUP BY c1;

Hint for query rewriting on materialized views

The query rewriting on materialized views is controlled by the MV_REWRITE and NO_MV_REWRITE hints. These hints take precedence over the system variable query_rewrite_enabled.

MV_REWRITE Hint

The syntax of the MV_REWRITE hint is as follows:

/*+ MV_REWRITE (@ queryblock [mv_name_list]) */

mv_name_list:
    mv_name [, mv_name ...]

When you use the MV_REWRITE hint alone, it skips the rules and cost checks for query rewriting on materialized views and directly applies the applicable rewrite. If you specify one or more materialized views after the hint, it skips the rules and cost checks and only uses the specified materialized views for rewriting, ignoring any unspecified materialized views.

When you use the MV_REWRITE hint to specify materialized views, you cannot force the use of a materialized view that does not have the ENABLE QUERY REWRITE clause. Additionally, you cannot force the use of a non-real-time materialized view when the system variable query_rewrite_integrity is set to enforced.

NO_MV_REWRITE Hint

The syntax of the NO_MV_REWRITE hint is as follows:

/*+ NO_MV_REWRITE (@ queryblock) */

The NO_MV_REWRITE hint disables query rewriting on materialized views and allows you to specify a query block.

Example of using the query rewriting on materialized views control hints

1. Create the base table tbl2.

   CREATE TABLE tbl2 (col1 INT, col2 INT);
   

2. Insert two rows into the tbl2 table.

   INSERT INTO tbl2 VALUES (1,2),(3,4);
   

   The return result is as follows:

   Query OK, 2 rows affected
   Records: 2  Duplicates: 0  Warnings: 0
   

3. Create the materialized view mv1_tbl2 and enable query rewriting on it.

   CREATE MATERIALIZED VIEW mv1_tbl2 NEVER REFRESH ENABLE QUERY REWRITE AS SELECT * FROM tbl2;
   

4. Create the materialized view mv2_tbl2 and enable query rewriting on it.

   CREATE MATERIALIZED VIEW mv2_tbl2 NEVER REFRESH ENABLE QUERY REWRITE AS SELECT * FROM tbl2 WHERE tbl2.col1 > 1;
   

5. Set the system variable query_rewrite_integrity to stale_tolerated.

   Note

   The MV_REWRITE and NO_MV_REWRITE hints take precedence over the system variable query_rewrite_enabled, so you do not need to set query_rewrite_enabled. However, you must set query_rewrite_integrity to stale_tolerated to use non-real-time materialized views for rewriting.

   SET query_rewrite_integrity = 'stale_tolerated';
   

6. Use the MV_REWRITE hint to attempt query rewriting on materialized views and skip the cost and rule checks. The following two queries will both use the mv1_tbl2 materialized view for rewriting.

   • /*+mv_rewrite*/ will attempt to use a materialized view that meets the rewrite conditions for rewriting. Once a suitable materialized view is found, subsequent materialized views will be ignored, and the cost and rule checks will be skipped.

     EXPLAIN SELECT /*+mv_rewrite*/ count(*), col1 FROM tbl2 WHERE tbl2.col1 > 1 GROUP BY col1;
     

     The return result is as follows:

     +----------------------------------------------------------------------------------------------+
     | Query Plan                                                                                   |
     +----------------------------------------------------------------------------------------------+
     | =====================================================                                        |
     | |ID|OPERATOR         |NAME    |EST.ROWS|EST.TIME(us)|                                        |
     | -----------------------------------------------------                                        |
     | |0 |HASH GROUP BY    |        |1       |3           |                                        |
     | |1 |└─TABLE FULL SCAN|MV1_TBL2|1       |3           |                                        |
     | =====================================================                                        |
     | Outputs & filters:                                                                           |
     | -------------------------------------                                                        |
     |   0 - output([T_FUN_COUNT(*)], [MV1_TBL2.COL1]), filter(nil), rowset=16                      |
     |       group([MV1_TBL2.COL1]), agg_func([T_FUN_COUNT(*)])                                     |
     |   1 - output([MV1_TBL2.COL1]), filter([MV1_TBL2.COL1 > cast(1, NUMBER(-1, -85))]), rowset=16 |
     |       access([MV1_TBL2.COL1]), partitions(p0)                                                |
     |       is_index_back=false, is_global_index=false, filter_before_indexback[false],            |
     |       range_key([MV1_TBL2.__pk_increment]), range(MIN ; MAX)always true                      |
     +----------------------------------------------------------------------------------------------+
     14 rows in set
     

   • /*+mv_rewrite(mv1_tbl2)*/ will attempt to use the mv2_tbl2 materialized view for rewriting and skip the cost and rule checks.

     EXPLAIN SELECT /*+mv_rewrite(mv2_tbl2)*/ count(*), col1 FROM tbl2 WHERE tbl2.col1 > 1 GROUP BY col1;
     

     The return result is as follows:

     +-------------------------------------------------------------------------+
     | Query Plan                                                              |
     +-------------------------------------------------------------------------+
     | =====================================================                   |
     | |ID|OPERATOR         |NAME    |EST.ROWS|EST.TIME(us)|                   |
     | -----------------------------------------------------                   |
     | |0 |HASH GROUP BY    |        |1       |3           |                   |
     | |1 |└─TABLE FULL SCAN|MV2_TBL2|1       |3           |                   |
     | =====================================================                   |
     | Outputs & filters:                                                      |
     | -------------------------------------                                   |
     |   0 - output([T_FUN_COUNT(*)], [MV2_TBL2.COL1]), filter(nil), rowset=16 |
     |       group([MV2_TBL2.COL1]), agg_func([T_FUN_COUNT(*)])                |
     |   1 - output([MV2_TBL2.COL1]), filter(nil), rowset=16                   |
     |       access([MV2_TBL2.COL1]), partitions(p0)                           |
     |       is_index_back=false, is_global_index=false,                       |
     |       range_key([MV2_TBL2.__pk_increment]), range(MIN ; MAX)always true |
     +-------------------------------------------------------------------------+
     14 rows in set
     

7. Although the query specifies using mv2_tbl2 for rewriting, the WHERE condition in the query does not meet the requirements, so mv2_tbl2 cannot be used for query rewriting. Therefore, no materialized view query rewriting is performed.

   EXPLAIN SELECT /*+mv_rewrite(mv2_tbl2)*/ count(*), col1 FROM tbl2 WHERE tbl2.col1 < 1 GROUP BY col1;
   

   The return result is as follows:

   +--------------------------------------------------------------------------------------+
   | Query Plan                                                                           |
   +--------------------------------------------------------------------------------------+
   | =================================================                                    |
   | |ID|OPERATOR         |NAME|EST.ROWS|EST.TIME(us)|                                    |
   | -------------------------------------------------                                    |
   | |0 |HASH GROUP BY    |    |1       |3           |                                    |
   | |1 |└─TABLE FULL SCAN|TBL2|1       |3           |                                    |
   | =================================================                                    |
   | Outputs & filters:                                                                   |
   | -------------------------------------                                                |
   |   0 - output([T_FUN_COUNT(*)], [TBL2.COL1]), filter(nil), rowset=16                  |
   |       group([TBL2.COL1]), agg_func([T_FUN_COUNT(*)])                                 |
   |   1 - output([TBL2.COL1]), filter([TBL2.COL1 < cast(1, NUMBER(-1, -85))]), rowset=16 |
   |       access([TBL2.COL1]), partitions(p0)                                            |
   |       is_index_back=false, is_global_index=false, filter_before_indexback[false],    |
   |       range_key([TBL2.__pk_increment]), range(MIN ; MAX)always true                  |
   +--------------------------------------------------------------------------------------+
   14 rows in set
   

8. Use the /*+ no_mv_rewrite*/ hint to disable query rewriting on materialized views.

   EXPLAIN SELECT /*+no_mv_rewrite*/ count(*), col1 FROM tbl2 WHERE tbl2.col1 > 1 GROUP BY col1;
   

   The return result is as follows:

   +--------------------------------------------------------------------------------------+
   | Query Plan                                                                           |
   +--------------------------------------------------------------------------------------+
   | =================================================                                    |
   | |ID|OPERATOR         |NAME|EST.ROWS|EST.TIME(us)|                                    |
   | -------------------------------------------------                                    |
   | |0 |HASH GROUP BY    |    |1       |3           |                                    |
   | |1 |└─TABLE FULL SCAN|TBL2|1       |3           |                                    |
   | =================================================                                    |
   | Outputs & filters:                                                                   |
   | -------------------------------------                                                |
   |   0 - output([T_FUN_COUNT(*)], [TBL2.COL1]), filter(nil), rowset=16                  |
   |       group([TBL2.COL1]), agg_func([T_FUN_COUNT(*)])                                 |
   |   1 - output([TBL2.COL1]), filter([TBL2.COL1 > cast(1, NUMBER(-1, -85))]), rowset=16 |
   |       access([TBL2.COL1]), partitions(p0)                                            |
   |       is_index_back=false, is_global_index=false, filter_before_indexback[false],    |
   |       range_key([TBL2.__pk_increment]), range(MIN ; MAX)always true                  |
   +--------------------------------------------------------------------------------------+
   14 rows in set
   

NO_PARALLEL Hint

The NO_PARALLEL hint specifies to disable query parallelism (i.e., set the parallelism to 1). Its opposite hint is the PARALLEL hint.

The NO_PARALLEL hint is equivalent to /*+ parallel(1)*/.

Syntax

/*+ NO_PARALLEL */

Example

The following query example uses the NO_PARALLEL hint to disable query parallelism.

SELECT /*+ no_parallel */ c1, sum(distinct c2) FROM t1 GROUP BY c1;

NO_QUERY_TRANSFORMATION Hint

The NO_QUERY_TRANSFORMATION hint disables any query rewriting for the current query.

Note that unlike the NO_REWRITE hint in the Query Block Hint, using the NO_QUERY_TRANSFORMATION hint prevents the use of Query Block hints to enable certain types of rewriting.

For more information about query rewriting, see Overview of query rewriting.

Syntax

/*+ NO_QUERY_TRANSFORMATION */

Example

The following query example uses the NO_QUERY_TRANSFORMATION hint to disable any query rewriting.

SELECT /*+ NO_QUERY_TRANSFORMATION */ *
FROM (SELECT * FROM t1) v WHERE v.c1 = 3;

OPT_PARAM Hint

OPT_PARAM Hint specifies optimizer-related configurations or system variables at the query level.

Syntax

/*+ OPT_PARAM ( parameter_name [,] parameter_value ) */

Parameters

• parameter_name: the name of a configuration or system variable.

• parameter_value: the value to be specified for the variable.

The OPT_PARAM Hint is applicable to the following parameters:

• rowsets_enabled: specifies whether to enable or disable vectorization. The data type is VARCHAR, and the value can be 'TRUE' or 'FALSE'. Enclose the value in single quotation marks (' ').

• rowsets_max_rows: specifies the number of rows to be returned in batches. The data type is INT, and the value ranges from 0 to 65535. Do not enclose the value in single quotation marks (' ').

• enable_newsort: specifies whether to enable or disable the newsort optimization. The data type is VARCHAR, and the value can be 'TRUE' or 'FALSE'. Enclose the value in single quotation marks (' ').

• use_part_sort_mgb: specifies whether to enable or disable the part sort merge group by. The data type is VARCHAR, and the value can be 'TRUE' or 'FALSE'. Enclose the value in single quotation marks (' ').

• enable_in_range_optimization: specifies whether to enable or disable the in optimization. The data type is VARCHAR, and the value can be 'TRUE' or 'FALSE'. Enclose the value in single quotation marks (' ').

• xsolapi_generate_with_clause: specifies whether to enable or disable the CTE extraction rewrite. The data type is VARCHAR, and the value can be 'TRUE' or 'FALSE'. Enclose the value in single quotation marks (' ').

• preserve_order_for_pagination: specifies whether to add an order by clause to a pagination query to preserve order or to prohibit adding the order by clause. The data type is VARCHAR, and the value can be 'TRUE' or 'FALSE'. Enclose the value in single quotation marks (' ').

• storage_card_estimation: specifies whether to use the row estimation based on the storage layer. The data type is VARCHAR, and the value can be 'TRUE' or 'FALSE'. Enclose the value in single quotation marks (' ').

• workarea_size_policy: specifies the strategy for manually or automatically adjusting the size of the SQL work area. The data type is VARCHAR, and the value can be 'MANUAL' for manual adjustment or 'AUTO' for automatic adjustment. Enclose the value in single quotation marks (' ').

• enable_rich_vector_format: specifies whether to enable or disable vectorization 2.0 (session-level parameter). The data type is VARCHAR, and the value can be 'TRUE' or 'FALSE'. Enclose the value in single quotation marks (' ').

• spill_compression_codec: specifies the compression algorithm for the operators that need to be temporarily materialized. The data type is VARCHAR, and the value can be NONE, LZ4, SNAPPY, ZLIB, or ZSTD, representing different compression algorithms. The default value is NONE, indicating no compression.

• inlist_rewrite_threshold: specifies the maximum number of constants that can trigger the rewrite of an inlist clause into a values statement. The data type is INT64, and the value ranges from 1 to 2147483647.

• orc_filter_pushdown_level: specifies the level at which to push down filter conditions for ORC external tables. The value of parameter_value can be:

  Note

  The OPT_PARAM Hint in OceanBase Database supports the orc_filter_pushdown_level parameter starting from V4.4.0.

  • 0: disables filter condition pushing down.
  • 1: pushes down filter conditions to the file level.
  • 2: pushes down filter conditions to the stripe level.
  • 3: pushes down filter conditions to the row index level.
  • 4: pushes down filter conditions to the encoding level.

• parquet_filter_pushdown_level: specifies the level at which to push down filter conditions for Parquet external tables. The value of parameter_value can be:

  Note

  The OPT_PARAM Hint in OceanBase Database supports the parquet_filter_pushdown_level parameter starting from V4.4.0.

  • 0: disables filter condition pushing down.
  • 1: pushes down filter conditions to the file level.
  • 2: pushes down filter conditions to the RowGroup level.
  • 3: pushes down filter conditions to the page level.
  • 4: pushes down filter conditions to the encoding level.

Examples

The following example uses the OPT_PARAM Hint to enable IN optimization for the current query by specifying the enable_in_range_optimization value.

SELECT /*+ opt_param('enable_in_range_optimization', 'true') */ *
from t1
where c1 in (1,2,3,4,5,...,1000)
and c2 in (1,2,3,4,5,...,1000);

PARALLEL Hint

As a global hint, the PARALLEL Hint specifies the parallelism or parallelism strategy for the current query.

In addition to being a global hint, the PARALLEL Hint can also be used as a query block hint to specify the parallelism at the table level. For more information, see the Parallel hint in Access Path Hints.

Syntax

/*+ PARALLEL ( AUTO | MANUAL | parallel_degree) */

Parameters

• AUTO or MANUAL: when AUTO or MANUAL is used as the parameter in the PARALLEL Hint, it specifies the parallelism strategy. For more information, see Parallelism strategies and their priorities.

• parallel_degree: when parallel_degree is used as the parameter in the PARALLEL Hint, it specifies the parallelism.

Examples

The following example uses the PARALLEL Hint to specify a parallelism of DOP = 8 or to enable Auto DOP.

SELECT /*+parallel(8)*/ c1, SUM(distinct c2) FROM t1 GROUP BY c1;

SELECT /*+parallel(auto)*/ c1, SUM(distinct c2) FROM t1 GROUP BY c1;

Note: Some scenarios do not support parallel execution. Even if you specify the PARALLEL hint to enable parallel execution, the execution plan may still disable parallel execution.

QUERY_TIMEOUT Hint

The QUERY_TIMEOUT hint specifies the execution timeout period for the current query.

Syntax

/*+ QUERY_TIMEOUT ( time_usec ) */

Parameters

• time_usec: specifies the query timeout period, in microseconds.

Examples

-- Specifies the query timeout period as 1 second. If the query is not completed within the specified timeout period, a timeout error is returned.
SELECT /*+ QUERY_TIMEOUT(1000000) */ *
FROM employees e
WHERE e.department_id = 1001;

READ_CONSISTENCY Hint

The READ_CONSISTENCY hint specifies the read consistency level for the current query.

Syntax

/*+ READ_CONSISTENCY(WEAK[STRONG]) */

Parameters

• WEAK: enables weak consistency, which means weak reads are enabled.

• STRONG: enables strong consistency, which means weak reads are disabled.

Examples

-- Uses the READ_CONSISTENCY hint and sets it to WEAK to enable weak reads for the query.
SELECT /*+ READ_CONSISTENCY(WEAK) */ *
FROM employees
WHERE employees.department_id = 1001;

Weak reads are supported when the isolation level is set to Repeatable Read or Serializable. Example:

obclient> SET transaction read only;
Query OK, 0 rows affected (0.001 sec)
obclient> SELECT /*+ read_consistency(weak)*/ * FROM t WHERE a=2;
+------+------+
| ID   | A    |
+------+------+
| NULL |    2 |
+------+------+
1 row in set (0.001 sec)

RESOURCE_GROUP Hint

The RESOURCE_GROUP hint forces the statement to use the specified resource group.

Syntax

The syntax of the RESOURCE_GROUP hint is as follows:

/*+ RESOURCE_GROUP ('resource_group_name') */

Parameters

• resource_group_name: specifies the name of the resource group to be used.

Examples

Here is an example of using the RESOURCE_GROUP hint:

obclient> SELECT /*+ RESOURCE_GROUP('big_group') */ * FROM t1;

If the resource group big_group does not exist, the current default resource group is used.

STAT Hint

The STAT hint specifies the operators whose outputs are to be traced in the query plan.

After the STAT hint is added, the query plan includes a MONITORING DUMP operator that directly outputs all data from the sort operator and prints the operator's execution time, output rows, and other information in the observer log after execution.

Syntax

/*+ STAT(TRACING_NUM_LIST) */

Parameters

• TRACING_NUM_LIST: specifies the IDs of the operators to be traced.

Examples

-- Displays the execution plans before and after adding the `/*+ STAT(0, 2) */` hint.
explain basic
SELECT /*+leading(t1) use_hash(t2)*/ * FROM t1, t2 WHERE t1.c1 = t2.c1;

Query Plan
===========================
|ID|OPERATOR         |NAME|
---------------------------
|0 |HASH JOIN        |    |
|1 |├─TABLE FULL SCAN|T1  |
|2 |└─TABLE FULL SCAN|T2  |
===========================

explain basic
SELECT /*+leading(t1) use_hash(t2) stat(0, 2)*/ *
FROM t1, t2 where t1.c1 = t2.c1;
Query Plan
===============================
|ID|OPERATOR             |NAME|
-------------------------------
|0 |MONITORING DUMP      |    |
|1 |└─HASH JOIN          |    |
|2 |  ├─TABLE FULL SCAN  |T1  |
|3 |  └─MONITORING DUMP  |    |
|4 |    └─TABLE FULL SCAN|T2  |
===============================

TRANS_PARAM Hint

The TRANS_PARAM hint specifies transaction-related parameters at the query level.

Syntax

/*+ TRANS_PARAM ['FORCE_EARLY_LOCK_FREE' , 'TRUE'] */

Parameters

Currently, only the FORCE_EARLY_LOCK_FREE parameter is supported at the transaction level.

• FORCE_EARLY_LOCK_FREE: specifies whether to enable early row lock release. Valid values: TRUE (enabled) and FALSE (disabled).

Notice: The parameter name and value must be enclosed in single quotation marks ('). If the value is a number, you can omit the quotation marks.

Examples

-- Uses the TRANS_PARAM hint and sets the parameter 'FORCE_EARLY_LOCK_FREE' to 'TRUE' to enable early row lock release at the transaction level.
SELECT /*+ TRANS_PARAM('FORCE_EARLY_LOCK_FREE' 'TRUE') */ *
FROM employees e
WHERE e.department_id = 1001;

TRACING Hint

The TRACING hint specifies the operators whose outputs are to be traced in the query plan.

The usage of the TRACING hint, the plan after adding the hint, and the usage of the STAT hint are the same.

The difference between using the TRACING hint and the STAT hint is that the MONITORING DUMP operator prints all of its output data in the observer log when the TRACING hint is used.

Syntax

/*+ TRACING(TRACING_NUM_LIST)*/

Parameters

• TRACING_NUM_LIST: specifies the IDs of the operators to be traced.

Examples

-- Uses the TRACING hint and sets the level to 1 to enable tracing for the current query.
SELECT /*+ TRACING(1) */ *
FROM employees e
WHERE e.department_id = 1001;

USE_PLAN_CACHE Hint

The USE_PLAN_CACHE hint specifies the plan cache strategy for the current query. For more information about plan cache, see Execution plan cache.

Syntax

/*+ USE_PLAN_CACHE ( NONE | DEFAULT ) */

Parameters

• NONE: specifies that the query does not use the plan cache.

• DEFAULT: specifies that the current query uses the system variable ob_enable_plan_cache to control the plan cache strategy.

Examples

-- Use the `NONE` parameter to specify that the query does not use the plan cache, and use the `DEFAULT` parameter to specify that the current query uses the system variable `ob_enable_plan_cache` to control the plan cache strategy.
SELECT /*+ USE_PLAN_CACHE(NONE) */ *
FROM employees e
WHERE e.department_id = 1001;

SELECT /*+ USE_PLAN_CACHE(DEFAULT) */ *
FROM employees e
WHERE e.department_id = 1001;

DISABLE_TRIGGER Hint

The DISABLE_TRIGGER hint temporarily disables triggers during the execution of the current DML statement. It applies to INSERT, UPDATE, DELETE, and DML operations on views (including INSTEAD OF triggers).

Syntax

You can use DISABLE_TRIGGER alone or follow it with a pair of parentheses, listing the trigger names (separated by commas) inside. Use parentheses, not brackets.

/*+ disable_trigger */
/*+ disable_trigger ( trigger_name [, trigger_name ... ] ) */

Parameters

trigger_name: an optional parameter specifying the name of the trigger to temporarily disable. You can specify one or more trigger names, separated by commas. If omitted, all triggers involved in the query are disabled by default.

Examples

-- Disable only the trigger named test_trigger1.
INSERT /*+ DISABLE_TRIGGER (test_trigger1) */ INTO test VALUES (2);

-- Disable all triggers related to this DML statement.
INSERT /*+ DISABLE_TRIGGER */ INTO test VALUES (3);

-- Specify multiple triggers.
UPDATE /*+ DISABLE_TRIGGER (tg_a, tg_b) */ t1 SET c = c + 1 WHERE id = 1;