Resetting Key Parameters During SQL Tuning

This section introduces key parameters of the primary database node that affect optimization of SQL statements in GaussDB. For details about the parameter configurations, see Configuring Running Parameters.

Table 1 Parameters of the primary database node

Parameter/Reference Value

Description

enable_nestloop=on

Specifies how the optimizer uses Nest Loop Join. If this parameter is set to on, the optimizer preferentially uses Nest Loop Join. If it is set to off, the optimizer preferentially uses other methods, if any.

NOTE:

If you only want to temporarily change the value of this parameter during the current database connection (that is, the current session), execute the following SQL statement:

          
               SET enable_nestloop to off;

By default, this parameter is set to on. Change the value as required. Generally, nested loop join has the poorest performance among the three JOIN methods (nested loop join, merge join, and hash join). You are advised to set this parameter to off.

enable_bitmapscan=on

Specifies whether the optimizer uses bitmap scanning. If the value is on, bitmap scanning is used. If the value is off, it is not used.

NOTE:

If you only want to temporarily change the value of this parameter during the current database connection (that is, the current session), execute the following SQL statement:

          
               SET enable_bitmapscan to off;

The bitmap scanning applies only in the query condition where a > 1 and b > 1 and indexes are created on columns a and b. During performance tuning, if the query performance is poor and bitmapscan operators are in the execution plan, set this parameter to off and check whether the performance is improved.

enable_hashagg=on

Specifies whether to enable the optimizer's use of Hash-aggregation plan types.

enable_hashjoin=on

Specifies whether to enable the optimizer's use of Hash-join plan types.

enable_mergejoin=on

Specifies whether to enable the optimizer's use of Hash-merge plan types.

enable_indexscan=on

Specifies whether to enable the optimizer's use of index-scan plan types.

enable_indexonlyscan=on

Specifies whether to enable the optimizer's use of index-only-scan plan types.

enable_seqscan=on

Specifies whether the optimizer uses bitmap scanning. It is impossible to suppress sequential scans entirely, but setting this variable to off encourages the optimizer to choose other methods if available.

enable_sort=on

Specifies the optimizer sorts. It is impossible to fully suppress explicit sorts, but setting this variable to off allows the optimizer to preferentially choose other methods if available.

rewrite_rule

Specifies whether the optimizer enables the LAZY_AGG and MAGIC_SET rewriting rules.

sql_beta_feature

Specifies whether the optimizer enables the SEL_SEMI_POISSON, SEL_EXPR_INSTR, PARAM_PATH_GEN, RAND_COST_OPT, PARAM_PATH_OPT, PAGE_EST_OPT, CANONICAL_PATHKEY, PARTITION_OPFUSION, PREDPUSH_SAME_LEVEL, PARTITION_FDW_ON, and DISABLE_BITMAP_COST_WITH_LOSSY_PAGES beta features.