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%. During sampling, default_statistics_target * 300 is used as the size of the random sampling. For example, if the default value is 100, 100 x 300 pages are read in a random sampling.

Type: USERSET

Value range: an integer ranging from -100 to 10000

• A larger positive number than the parameter 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 optimize the query plan.
• 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 autoanalyze function does not support percentage sampling. The sampling uses the default value of this parameter.
• If this parameter is set to a positive value, you must have the ANALYZE permission to execute ANALYZE.
• If this parameter is set to a negative value, that is, percentage sampling, you need to be granted the ANALYZE and SELECT permissions to execute ANALYZE.

Default value: 100

random_function_version

Parameter description: Specifies the random function version selected by ANALYZE during data sampling. This feature is supported only in 8.1.2 or later.

Type: USERSET

Value range: enumerated values

• The value 0 indicates that the random function provided by the C standard library is used.
• The value 1 indicates that the optimized and enhanced random function is used.

Default value: 0

constraint_exclusion

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

Type: USERSET

Value range: enumerated values

• on indicates the constraints for all tables are examined.
• off: No constraints are examined.
• partition indicates that only constraints for inherited child tables and UNION ALL subqueries are examined.
  

  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

Currently, this parameter is set to on by default to partition tables. If this parameter is set to on, extra planning is imposed on simple queries, which has no benefits. 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.

Type: USERSET

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

Smaller values than the default value 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.

Type: USERSET

Value range: an integer ranging from 1 to INT_MAX

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.

Type: USERSET

Value range: an integer ranging from 1 to INT_MAX

• 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 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. OPTIMIZE_PLAN indicates the optimal plan, the cost of which is estimated using the dynamic planning algorithm, and its value is 0. RANDOM_PLAN indicates the plan that is randomly generated. If plan_mode_seed is set to -1, you do not need to specify the value of the seed identifier. Instead, the optimizer generates a random integer ranging from 1 to 2147483647, and then generates a random execution plan based on this random number. If plan_mode_seed is set to an integer ranging from 1 to 2147483647, you need to specify the value of the seed identifier, and the optimizer generates a random execution plan based on the seed value.

Type: USERSET

Value range: an integer ranging from -1 to 2147483647

Default value: 0

• If plan_mode_seed is set to RANDOM_PLAN, the optimizer generates different random execution plans, which may not be the optimal. 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.

enable_hdfs_predicate_pushdown

Parameter description: Specifies whether the function of pushing down predicates the native data layer is enabled.

Type: SUSET

Value range: Boolean

• on indicates this function is enabled.
• off indicates this function is disabled.

Default value: on

enable_random_datanode

Parameter description: Specifies whether the function that random query about DNs in the replication table is enabled. A complete data table is stored on each DN for random retrieval to release the pressure on nodes.

Type: USERSET

Value range: Boolean

• on: This function is enabled.
• off: This function is disabled.

Default value: on

hashagg_table_size

Parameter description: Specifies the hash table size during HASH AGG execution.

Type: USERSET

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

Default value: 0

enable_codegen

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

Type: USERSET

Value range: Boolean

• on indicates code optimization can be enabled.
• off indicates code optimization cannot be enabled.
  

  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 codegen-based.

Type: USERSET

Value range: enumerated values

• partial indicates that you can still call the LLVM dynamic optimization strategy using the codegen framework of an expression even if functions that are not codegen-based exist in the expression.
• pure indicates that the LLVM dynamic optimization strategy can be called only when all functions in an expression can be codegen-based.
  

  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.

Type: USERSET

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.

Type: USERSET

Value range: an integer ranging from 0 to INT_MAX

Default value: 10000

enable_constraint_optimization

Parameter description: Specifies whether the informational constraint optimization execution plan can be used for an HDFS foreign table.

Type: SUSET

Value range: Boolean

• on indicates the plan can be used.
• off indicates the plan cannot be used.

Default value: on

enable_bloom_filter

Parameter description: Specifies whether the BloomFilter optimization is used.

Type: USERSET

Value range: Boolean

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

Default value: on

Scenario: If in a HASH JOIN, the thread of the foreign table contains HDFS tables or column-store tables, the Bloom filter is triggered.

Constraints:

1. Only INNER JOIN, SEMI JOIN, RIGHT JOIN, RIGHT SEMI JOIN, RIGHT ANTI JOIN and RIGHT ANTI FULL JOIN are supported.
2. The number of rows in the internal table in the join cannot exceed 50,000.
3. JOIN condition of the internal table: It cannot be an expression for HDFS internal or foreign tables. It can be an expression for column-store tables, but only at the non-join layer.
4. The join condition of the foreign table must be simple column join.
5. When the join conditions of the internal and foreign tables (HDFS) are both simple column joins, the estimated data that can be removed at the plan layer must be over 1/3.
6. Joined columns cannot contain NULL values.
7. Data is not flushed to disks at the JOIN layer.
8. Data type:
   • HDFS internal and foreign tables support SMALLINT, INTEGER, BIGINT, REAL/FLOAT4, DOUBLE PRECISION/FLOAT8, CHAR(n)/CHARACTER(n)/NCHAR(n), VARCHAR(n)/CHARACTER VARYING(n), CLOB and TEXT.
   • Column-store tables support SMALLINT, INTEGER, BIGINT, OID, "char", CHAR(n)/CHARACTER(n)/NCHAR(n), VARCHAR(n)/CHARACTER VARYING(n), NVARCHAR2(n), CLOB, TEXT, DATE, TIME, TIMESTAMP and TIMESTAMPTZ. The collation of the character type must be C.

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.

Type: USERSET

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 allow automatic statistics collection for a table that has no statistics or a table whose amount of data modification reaches the threshold for triggering ANALYZE when a plan is generated. In this case, AUTOANALYZE cannot be triggered for foreign tables or temporary tables with the ON COMMIT [DELETE ROWS|DROP] option. To collect statistics, you need to 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.

If the amount of data modification reaches the threshold for triggering ANALYZE, the amount of data modification exceeds autovacuum_analyze_threshold + autovacuum_analyze_scale_factor * reltuples. reltuples indicates the estimated number of rows in the table recorded in pg_class.

Type: SUSET

Value range: Boolean

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

Default value: on

query_dop

Parameter description: Specifies the user-defined degree of parallelism.

Type: USERSET

Value range: an integer ranging from -64 to 64.

[1, 64]: Fixed SMP is enabled, and the system will use the specified degree.

0: SMP adaptation function is enabled. The system dynamically selects the optimal parallelism degree [1,8] (x86 platforms) or [1,64] (Kunpeng platforms) for each query based on the resource usage and query plans.

[-64, -1]: SMP adaptation is enabled, and the system will dynamically select a degree from the limited range.

• For TP services that mainly involve short queries, if services cannot be optimized through lightweight CNs or statement delivery, it will take a long time to generate an SMP plan. You are advised to set query_dop to 1. For AP services with complex statements, you are advised to set query_dop to 0.
• After enabling concurrent queries, ensure you have sufficient CPU, memory, network, and I/O resources to achieve the optimal performance.
• To prevent performance deterioration caused by an overly large value of query_dop, the system calculates the maximum number of available CPU cores for a DN and uses the number as the upper limit for this parameter. If the value of query_dop is greater than 4 and also the upper limit, the system resets query_dop to the upper limit.

Default value: 1

query_dop_ratio

Parameter description: Specifies the DOP multiple used to adjust the optimal DOP preset in the system when query_dop is set to 0. That is, DOP = Preset DOP x query_dop_ratio (ranging from 1 to 64). If this parameter is set to 1, the DOP cannot be adjusted.

Type: USERSET

Value range: a floating point number ranging from 0 to 64

Default value: 1

debug_group_dop

Parameter description: Specifies the unified DOP parallelism degree allocated to the groups that use the Stream operator as the vertex in the generated execution plan when the value of query_dop is 0. This parameter is used to manually specify the DOP for specific groups for performance optimization. Its format is G1,D1,G2,D2,...,, where G1 and G2 indicate the group IDs that can be obtained from logs and D1 and D2 indicate the specified DOP values and can be any positive integers.

Type: USERSET

Value range: a string

Default value: empty

This parameter is used only for internal optimization and cannot be set. You are advised to use the default value.

enable_analyze_check

Parameter description: Checks whether statistics were collected about tables whose reltuples and relpages are shown as 0 in pg_class during plan generation. This parameter is no longer used in cluster versions 8.1.3 and later, but is reserved for compatibility with earlier versions. The setting of this parameter does not take effect.

Type: SUSET

Value range: Boolean

• on enables the check.
• off disables the check.

Default value: on

enable_sonic_hashagg

Parameter description: Specifies whether to use the Hash Agg operator for column-oriented hash table design when certain constraints are met.

Type: USERSET

Value range: Boolean

• on indicates that the Hash Agg operator is used for column-oriented hash table design when certain constraints are met.
• off indicates that the Hash Agg operator is not used for column-oriented hash table design.

• If enable_sonic_hashagg is enabled and certain constraints are met, the Hash Agg operator will be used for column-oriented hash table design, and the memory usage of the operator can be reduced. However, in scenarios where the code generation technology (enabled by enable_codegen) can significantly improve performance, 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.

Default value: on

enable_sonic_hashjoin

Parameter description: Specifies whether to use the Hash Join operator for column-oriented hash table design when certain constraints are met.

Type: USERSET

Value range: Boolean

• on indicates that the Hash Join operator is used for column-oriented hash table design when certain constraints are met.
• off indicates that the Hash Join operator is not used for column-oriented hash table design.

• Currently, the parameter can be used only for Inner Join.
• If enable_sonic_hashjoin is enabled, the memory usage of 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.

Default value: on

enable_sonic_optspill

Parameter description: Specifies whether to optimize the number of hash join or hash agg files flushed to disks in the sonic scenario. This parameter takes effect only when enable_sonic_hashjoin or enable_sonic_hashagg is enabled.

Type: USERSET

Value range: Boolean

• on indicates that the number of files flushed to disks is optimized.
• off indicates that the number of files flushed to disks is not optimized.

For the hash join or hash agg operator that meets the sonic criteria, if this parameter is set to off, one file is flushed to disks for each column. If this parameter is set to on and the data types of different columns are similar, only one file (a maximum of five files) will be flushed to disks.

Default value: on

expand_hashtable_ratio

Parameter description: Specifies the expansion ratio used to resize the hash table during the execution of the Hash Agg and Hash Join operators.

Type: USERSET

Value range: a floating point number of 0 or ranging from 0.5 to 10

• Value 0 indicates that the hash table is adaptively expanded based on the current memory size.
• The value ranging from 0.5 to 10 indicates the multiple used to expand the hash table. Generally, a larger hash table delivers better performance but occupies more memory space. If the memory space is insufficient, data may be spilled to disks in advance, causing performance deterioration.

Default value: 0

plan_cache_mode

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

Type: USERSET

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.

• 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 a prepare statement where parameters in the execute statement is embedded in the prepare statement. 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 for 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 solution 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 parameter field. When some bound parameters are used, the plan execution performance is poor.

Default value: auto

wlm_query_accelerate

Parameter description: Specifies whether the query needs to be accelerated when short query acceleration is enabled.

Type: USERSET

Value range: an integer ranging from –1 to 1

• -1: indicates that short queries are controlled by the fast lane, and the long queries are controlled by the slow lane.
• 0: indicates that queries are not accelerated. Both short and long queries are controlled by the slow lane.
• 1: indicates that queries are accelerated. Both short queries and long queries are controlled by the fast lane.

Default value: –1

show_unshippable_warning

Parameter description: Specifies whether to print the alarm for the statement pushdown failure to the client.

Type: USERSET

Value range: Boolean

• on: Records the reason why the statement cannot be pushed down in a WARNING log and prints the log to the client.
• off: Logs the reason why the statement cannot be pushed down only.

Default value: off

hashjoin_spill_strategy

Parameter description: specifies the hash join policy for flushing data to disks. This feature is supported in 8.1.2 or later.

Type: USERSET

Value range: The value is an integer ranging from 0 to 4.

• 0: If the size of the inner table is large and cannot be partitioned after data is flushed to disks for multiple times, the system attempts to place the outer table in the available memory of the database to create a hash table. If both the inner and outer tables are large, a nested loop join is performed.
• 1: If the size of the inner table is large and cannot be partitioned after data is flushed to disks for multiple times, the system attempts to place the outer table in the available memory of the database to create a hash table. If both the inner and outer tables are large, a hash join is forcibly performed.
• 2: If the size of the inner table is large and cannot be partitioned after data is flushed to disks for multiple times, a hash join is forcibly performed.
• 3: If the size of the inner table is large and cannot be partitioned after data is flushed to disks for multiple times, the system attempts to place the outer table in the available memory of the database to create a hash table. If both the inner and outer tables are large, an error is reported.
• 4: If the size of the inner table is large and cannot be partitioned after data is flushed to disks for multiple times, an error is reported.

• This parameter is valid only for a vectorized hash join operator.
• If the number of distinct values is small and the data volume is large, data may fail to be flushed to disks. As a result, the memory usage is too high and the memory is out of control. If this parameter is set to 0, the system attempts to swap the inner and outer tables or perform a nested loop join to prevent this problem. However, a nested loop join may deteriorate performance in some scenarios.
• The value 0 does not take effect for a vectorized full join, and the behavior is the same as that of the value 1. The system attempts to create a hash table only for the outer table and does not perform a nested loop join.

Default value: 0

max_streams_per_query

Parameter description: Controls the number of Stream nodes in a query plan. (This parameter is supported only in 8.1.3.200 and later cluster versions.)

Type: SUSET

Value range: an integer ranging from –1 to 10000.

• -1 indicates that the number of Stream nodes in the query plan is not limited.
• A value within the range 0 to 10000 indicates that when the number of Stream nodes in the query plan exceeds the specified value, an error is reported and the query plan will not be executed.

• This parameter controls only the Stream nodes on DNs and does not control the Gather nodes on the CN.
• This parameter does not affect the EXPLAIN query plan, but affects EXPLAIN ANALYZE and EXPLAIN PERFORMANCE.

Default value: –1