Genetic Query Optimizer

geqo

Parameter description: Specifies whether to enable the genetic query optimization.

If this parameter is modified by running the gs_guc reload command and the connection of a session on the current node is not from the client but from another node in the cluster to which the node belongs, this parameter does not take effect immediately on the session after the gs_guc reload command is executed. The setting takes effect only after the connection node is disconnected and then reconnected.

Parameter type: Boolean.

Unit: none

Value range:

• on: used.
• off: not used.

Default value: on

Setting method: This is a USERSET parameter. Set it based on instructions in Table 1.

Setting suggestion: It is best not to turn it off in execution. geqo_threshold provides more subtle control of GEQO.

geqo_threshold

Parameter description: Specifies the number of FROM items. Genetic query optimization is used to plan queries when the number of statements executed is greater than this value.

• For simpler queries it is best to use the regular, exhaustive-search planner, but for queries with many tables it is better to use GEQO to manage the queries.
• A FULL OUTER JOIN construct counts as only one FROM item.

Parameter type: integer.

Unit: none

Value range: 2 to 2147483647.

Default value: 12

Setting method: This is a USERSET parameter. Set it based on instructions in Table 1.

Setting suggestion: none

geqo_effort

Parameter description: Controls the trade-off between planning time and query plan quality in GEQO.

Parameter type: integer.

Unit: none

Value range: 1 to 10

Default value: 5

• geqo_effort does not perform operations directly. This parameter is only used to compute the default values for the other variables that influence GEQO behavior. If you prefer, you can manually set the other parameters instead.
• Larger values increase the time spent in query planning, but also increase the probability that an efficient query plan is chosen.

geqo_pool_size

Parameter description: Controls the pool size used by GEQO, that is, the number of individuals in the genetic population.

Parameter type: integer.

Unit: none

Value range: 0 to 2147483647

Default value: 0.

If this parameter is set to 0, GaussDB selects a proper value based on geqo_effort and the number of tables. In other cases, the value of the parameter is at least 2, and the useful value typically ranges from 100 to 1000.

Setting method: This is a USERSET parameter. Set it based on instructions in Table 1.

Setting suggestion: none

geqo_generations

Parameter description: Specifies the number of iterations of the GEQO.

Parameter type: integer.

Unit: none

Value range: 0 to 2147483647

Default value: 0.

If it is set to 0, a suitable value is chosen based on geqo_pool_size. In other cases, the value of this parameter is at least 1, and useful values are typically from 100 to 1000.

Setting method: This is a USERSET parameter. Set it based on instructions in Table 1.

Setting suggestion: none

geqo_selection_bias

Parameter description: Specifies the selection bias used by GEQO. The selection bias is the selective pressure within the population.

Parameter type: floating-point.

Unit: none

Value range: 1.5–2.0

Default value: 2

Setting method: This is a USERSET parameter. Set it based on instructions in Table 1.

Setting suggestion: none

geqo_seed

Parameter description: Specifies the initial value of the random number generator used by GEQO to select random paths through the join order search space.

Parameter type: floating-point.

Unit: none

Value range: 0.0 to 1.0

Default value: 0.

Varying the value changes the set of join paths explored, and may result in a better or worse best path being found.

Setting method: This is a USERSET parameter. Set it based on instructions in Table 1.

Setting suggestion: none