Other Optimizer Options

default_statistics_target

Parameter description: Specifies the default statistics target for table columns without a column-specific target set via ALTER TABLE SET STATISTICS. If this parameter is set to a positive number, it indicates the number of samples of statistics information. If this parameter is set to a negative number, percentage is used to set the statistic target. The negative number converts to its corresponding percentage, for example, -5 means 5%.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from -100 to 10000

NOTICE:

• A larger positive number than the default value increases the time required to do ANALYZE, but might improve the quality of the optimizer's estimates.
• Changing settings of this parameter may result in performance deterioration. If query performance deteriorates, you can: 1. Restore to the default statistics. 2. Use hints to force the optimizer to use the optimal query plan. For details, see Hint-based Tuning.
• If this parameter is set to a negative value, the number of samples is greater than or equal to 2% of the total data volume, and the number of records in user tables is less than 1.6 million, the time taken by running ANALYZE will be longer than when this parameter uses its default value.
• If this parameter is set to a negative value, the auto-analyze function is disabled.

Default value: 100

constraint_exclusion

Parameter description: Controls the query optimizer's use of table constraints to optimize queries.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: enumerated values

• on indicates that constraints for all tables are examined.

• off indicates that constraints for any table are not examined.

• partition indicates that only constraints for inheritance child tables and UNION ALL subqueries are examined.

  NOTICE:

  When constraint_exclusion is set to on, the optimizer compares query conditions with the table's CHECK constraints, and omits scanning tables for which the conditions contradict the constraints.

Default value: partition

NOTE:

Currently, constraint exclusion is enabled by default only for cases that are often used to implement table partitioning. Turning this feature on for all tables imposes extra planning on simple queries, and provides no benefit for simple queries. If you have no partitioned tables, set it to off.

cursor_tuple_fraction

Parameter description: Specifies the optimizer's estimated fraction of a cursor's rows that are retrieved.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: a floating point number ranging from 0.0 to 1.0

NOTICE:

Smaller values of this setting bias the optimizer towards using fast start plans for cursors, which will retrieve the first few rows quickly while perhaps taking a long time to fetch all rows. Larger values put more emphasis on the total estimated time. At the maximum setting of 1.0, cursors are planned exactly like regular queries, considering only the total estimated time and how soon the first rows might be delivered.

Default value: 0.1

from_collapse_limit

Parameter description: Specifies whether the optimizer merges sub-queries into upper queries based on the resulting FROM list. The optimizer merges sub-queries into upper queries if the resulting FROM list would have no more than this many items.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 1 to INT_MAX

NOTICE:

Smaller values reduce planning time but may lead to inferior execution plans.

Default value: 8

join_collapse_limit

Parameter description: Specifies whether the optimizer rewrites JOIN constructs (except FULL JOIN) into lists of FROM items based on the number of the items in the result list.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 1 to INT_MAX

NOTICE:

• Setting this parameter to 1 prevents join reordering. As a result, the join order specified in the query will be the actual order in which the relations are joined. The query optimizer does not always choose the optimal join order. Therefore, advanced users can temporarily set this variable to 1, and then specify the join order they desire explicitly.
• Smaller values reduce planning time but may lead to inferior execution plans.

Default value: 8

plan_mode_seed

Parameter description: This is a commissioning parameter. Currently, it supports only OPTIMIZE_PLAN and RANDOM_PLAN. The value 0 (for OPTIMIZE_PLAN) indicates the optimized plan using the dynamic planning algorithm. Other values are for RANDOM_PLAN, which indicates that the plan is randomly generated. -1 indicates that users do not specify the value of the seed identifier. In this case, the optimizer generates a random integer from 1 to 2147483647 and a random execution plan based on the generated integer. A value from 1 to 2147483647 is regarded as the seed identifier, based on which the optimizer generates a random execution plan.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from -1 to 2147483647

Default value: 0

NOTICE:

• If plan_mode_seed is set to RANDOM_PLAN, the optimizer generates a random execution plan that may not be the optimal one. Therefore, to guarantee the query performance, the default value 0 is recommended during upgrade, scale-out, scale-in, and O&M.
• If this parameter is not set to 0, the specified hint will not be used.

hashagg_table_size

Parameter description: Specifies the hash table size during the execution of the HASH JOIN operation.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 0 to INT_MAX/2

Default value: 0

enable_codegen

Parameter description: Specifies whether code optimization is enabled. Currently, the code optimization uses the LLVM optimization.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that code optimization is enabled.

• off indicates that code optimization is disabled.

  NOTICE:

  Currently, the LLVM optimization only supports the vectorized executor and SQL on Hadoop features. You are advised to set this parameter to off in other cases.

Default value: on

codegen_strategy

Parameter description: Specifies the codegen optimization strategy that is used when an expression is converted to be codegen-based.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: enumerated values

• partial indicates that even if functions that are not codegen-based exist in an expression, you can still call the LLVM dynamic optimization strategy by using the entire codegen framework of the expression.

• pure indicates that only when all functions in an expression can be codegen-based, the LLVM dynamic optimization strategy can be called.

  NOTICE:

  In the scenario where query performance reduces after the codegen function is enabled, you can set this parameter to pure. In other scenarios, do not change the default value partial of this parameter.

Default value: partial

enable_codegen_print

Parameter description: Specifies whether the LLVM IR function can be printed in logs.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the LLVM IR function can be printed in logs.
• off indicates that the LLVM IR function cannot be printed in logs.

Default value: off

codegen_cost_threshold

Parameter description: The LLVM compilation takes some time to generate executable machine code. Therefore, LLVM compilation is beneficial only when the actual execution cost is more than the sum of the code required for generating machine code and the optimized execution cost. This parameter specifies a threshold. If the estimated execution cost exceeds the threshold, LLVM optimization is performed.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 0 to 2147483647

Default value: 10000

enable_bloom_filter

Parameter description: Specifies whether the BloomFilter optimization can be used. This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the BloomFilter optimization can be used.
• off indicates that the BloomFilter optimization cannot be used.

Default value: on

enable_extrapolation_stats

Parameter description: Specifies whether the extrapolation logic is used for data of DATE type based on historical statistics. The logic can increase the accuracy of estimation for tables whose statistics are not collected in time, but will possibly provide an overlarge estimation due to incorrect extrapolation. Enable the logic only in scenarios where the data of DATE type is periodically inserted. This parameter is a SUSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the extrapolation logic is used for data of DATE type based on historical statistics.
• off indicates that the extrapolation logic is not used for data of DATE type based on historical statistics.

Default value: off

autoanalyze

Parameter description: Specifies whether to automatically collect statistics on tables that have no statistics when a plan is generated. autoanalyze cannot be used for foreign or temporary tables. To collect statistics, manually perform the ANALYZE operation. If an exception occurs in the database during the execution of autoanalyze on a table, after the database is recovered, the system may still prompt you to collect the statistics of the table when you run the statement again. In this case, manually perform the ANALYZE operation on the table to synchronize statistics. This parameter is a SUSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the table statistics are automatically collected.
• off indicates that the table statistics are not automatically collected.

Default value: off

enable_analyze_check

Parameter description: Checks whether statistics were collected about tables whose reltuples and relpages are displayed as 0 in pg_class during plan generation.

This parameter is a SUSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the tables will be checked.
• off indicates that the tables will not be checked.

Default value: off

enable_sonic_hashagg

Parameter description: Specifies whether to use the hash aggregation operator designed for column-oriented hash tables when certain constraints are met.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the hash aggregation operator designed for column-oriented hash tables is used when certain constraints are met.
• off indicates that the hash aggregation operator designed for column-oriented hash tables is not used.

NOTE:

• When the hash aggregation operator designed for column-oriented hash tables is used, the memory usage of the query can be reduced. However, in scenarios when enable_codegen is set to on and the performance is significantly improved, the performance of the operator may deteriorate.
• If enable_sonic_hashagg is set to on, when certain constraints are met, the hash aggregation operator designed for column-oriented hash tables is used and its name is displayed as Sonic Hash Aggregation in the output of the Explain Analyze/Performance operation. When the constraints are not met, the operator name is displayed as Hash Aggregation. For details, see Description).

Default value: on

enable_sonic_hashjoin

Parameter description: Specifies whether to use the hash join operator designed for column-oriented hash tables when certain constraints are met.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the hash join operator designed for column-oriented hash tables is used when certain constraints are met.
• off indicates that the hash join operator designed for column-oriented hash tables is not used.

NOTE:

• Currently, the parameter can be used only for Inner Join.
• If enable_sonic_hashjoin is enabled, the memory usage of query using the Hash Inner operator can be reduced. However, in scenarios where the code generation technology can significantly improve performance, the performance of the operator may deteriorate.
• If enable_sonic_hashjoin is set to on, when certain constraints are met, the hash join operator designed for column-oriented hash tables is used and its name is displayed as Sonic Hash Join in the output of the Explain Analyze/Performance operation. When the constraints are not met, the operator name is displayed as Hash Join. For details, see Description.

Default value: on

enable_sonic_optspill

Parameter description: Specifies whether to optimize the number of files to be written to disks for the Hash Join operator designed for column-oriented hash tables. If this parameter is set to on, the number of files written to disk does not increase significantly when the Hash Join operator writes a large number of files to disk.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the optimization is enabled.
• off indicates that the optimization is disabled.

Default value: on

log_parser_stats

Parameter description: Specifies whether the optimizer outputs the performance logs of the parser module.

This parameter is a SUSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the performance logs are output.
• off indicates that the performance logs are not output.

Default value: off

log_planner_stats

Parameter description: Specifies whether the optimizer outputs the performance logs of the planner module.

This parameter is a SUSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the performance logs are output.
• off indicates that the performance logs are not output.

Default value: off

log_executor_stats

Parameter description: Specifies whether the optimizer outputs the performance logs of the executor module.

This parameter is a SUSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the performance logs are output.
• off indicates that the performance logs are not output.

Default value: off

log_statement_stats

Parameter description: Specifies whether the optimizer outputs the performance logs of a statement.

This parameter is a SUSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the performance logs are output.
• off indicates that the performance logs are not output.

Default value: off

plan_cache_mode

Parameter description: Specifies the policy for generating an execution plan in the prepare statement.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: enumerated values

• auto indicates that the custom plan or generic plan is selected by default.
• force_generic_plan indicates that the generic plan is forcibly used.
• force_custom_plan indicates that the custom plan is forcibly used.

NOTE:

• This parameter is valid only for the prepare statement. It is used when the parameterized field in the prepare statement has severe data skew.
• custom plan is a plan generated after you run the prepare statement where parameters in the execute statement is embedded. The custom plan generates a plan based on specific parameters in the execute statement. This scheme generates a preferred plan based on specific parameters each time and has good execution performance. The disadvantage is that the plan needs to be regenerated before each execution, resulting in a large amount of repeated optimizer overhead.
• generic plan is a plan generated after you run the prepare statement. The plan policy binds parameters to the plan when you run the execute statement and execute the plan. The advantage of this scheme is that repeated optimizer overheads can be avoided in each execution. The disadvantage is that the plan may not be optimal when data skew occurs for the bound parameters and may result in poor plan execution performance.

Default value: auto

enable_hypo_index

Parameter description: Controls whether the optimizer considers virtual indexes when executing the EXPLAIN command.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

• on indicates that the performance logs are output.
• off indicates that the performance logs are not output.

Default value: off