Help Center> GaussDB> Distributed_2.x> GUC Parameters> Query Planning> Optimizer Cost Constants
Updated on 2023-10-23 GMT+08:00
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Optimizer Cost Constants

This section describes the optimizer cost constants. The cost variables described here are measured on an arbitrary scale. Only their relative values matter, therefore scaling them all up or down by the same factor will result in no change in the optimizer's choices. By default, these cost variables are based on the cost of sequential page fetches, that is, seq_page_cost is conventionally set to 1.0 and the other cost variables are set with reference to the parameter. However, you can use a different scale, such as actual execution time in milliseconds.

seq_page_cost

Parameter description: Specifies the optimizer's estimated cost of a disk page fetch that is part of a series of sequential fetches.

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

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

Default value: 1

random_page_cost

Parameter description: Specifies the optimizer's estimated cost of an out-of-sequence disk page fetch.

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

Although the server allows you to set random_page_cost to a value less than that of seq_page_cost, it is not physically sensitive to do so. However, setting them equal makes sense if the database is entirely cached in RAM, because in that case there is no penalty for fetching pages out of sequence. Also, in a heavily-cached database you should lower both values relative to the CPU parameters, since the cost of fetching a page already in RAM is much smaller than it would normally be.

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

Default value: 4

  • This value can be overwritten for tables and indexes in a particular tablespace by setting the tablespace parameter of the same name.
  • Reducing this value relative to seq_page_cost will cause the system to prefer index scans and raising it will make index scans relatively more expensive. You can increase or decrease both values together to change the disk I/O costs relative to CPU costs.

cpu_tuple_cost

Parameter description: Specifies the optimizer's estimated cost of processing each row during a query.

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

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

Default value: 0.01

cpu_index_tuple_cost

Parameter description: Specifies the optimizer's estimated cost of processing each index entry during an index scan.

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

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

Default value: 0.005

cpu_operator_cost

Parameter description: Specifies the optimizer's estimated cost of processing each operator or function executed during a query.

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

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

Default value: 0.0025

effective_cache_size

Parameter description: Specifies the optimizer's assumption about the effective size of the disk cache that is available to a single query.

Set this parameter based on the following factors: the GaussDB's shared buffer space, the kernel's disk buffer space, and the estimated number of concurrent queries on different tables that share the available space.

This parameter does not affect the size of the shared memory allocated during actual GaussDB running. It is used only for estimation in the plan generation phase. The value is in the unit of disk page. Usually the size of each page is 8192 bytes.

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

Value range: an integer ranging from 1 to 2147483647. The unit is 8 KB.

Default value:

Independent deployment:

CN: 2 GB (60-core CPU/480 GB memory); 1 GB (32-core CPU/256 GB memory and 16-core CPU/128 GB memory); 512 MB (8-core CPU/64 GB memory); 256 MB (4-core CPU/32 GB memory); 128 MB (4-core CPU/16 GB memory)

DN: 70 GB (60-core CPU/480 GB memory); 38 GB (32-core CPU/256 GB memory); 20 GB (16-core CPU/128 GB memory); 8 GB (8-core CPU/64 GB memory); 4 GB (4-core CPU/32 GB memory); 2 GB (4-core CPU/16 GB memory)

Finance edition (standard):

CN: 1 GB (128-core CPU/1024 GB memory, 104-core CPU/1024 GB memory, 96-core CPU/1024 GB memory, 96-core CPU/768 GB memory, 80-core CPU/640 GB memory, 72-core CPU/576 GB memory, 64-core CPU/512 GB memory, and 60-core CPU/480 GB memory); 512 MB (32-core CPU/256 GB memory and 16-core CPU/128 GB memory); 256 MB (8-core CPU/64 GB memory)

DN: 70 GB (128-core CPU/1024 GB memory, 104-core CPU/1024 GB memory, and 96-core CPU/1024 GB memory); 50 GB (96-core CPU/768 GB memory); 40 GB (80-core CPU/640 GB memory and 72-core CPU/576 GB memory); 35 GB (64-core CPU/512 GB memory); 30 GB (60-core CPU/480 GB memory); 15 GB (32-core CPU/256 GB memory); 8 GB (16-core CPU/128 GB memory); 4 GB (8-core CPU/64 GB memory)

Enterprise edition:

CN: 1 GB (128-core CPU/1024 GB memory, 104-core CPU/1024 GB memory, 96-core CPU/1024 GB memory, 96-core CPU/768 GB memory, 80-core CPU/640 GB memory, 80-core CPU/512 GB memory, 72-core CPU/576 GB memory, 64-core CPU/512 GB memory, and 60-core CPU/480 GB memory); 512 MB (32-core CPU/256 GB memory and 16-core CPU/128 GB memory); 256 MB (8-core CPU/64 GB memory)

DN: 50 GB (128-core CPU/1024 GB memory, 104-core CPU/1024 GB memory, and 96-core CPU/1024 GB memory); 38 GB (96-core CPU/768 GB memory); 32 GB (80-core CPU/640 GB memory); 28 GB (72-core CPU/576 GB memory); 25 GB (80-core CPU/512 GB memory and 64-core CPU/512 GB memory); 24 GB (60-core CPU/480 GB memory); 12 GB (32-core CPU/256 GB memory); 5 GB (16-core CPU/128 GB memory); 3 GB (8-core CPU/64 GB memory)

Finance edition (data computing):

CN: 1 GB (96-core CPU/768 GB memory); 512 MB (72-core CPU/576 GB memory and 64-core CPU/512 GB memory); 256 MB (32-core CPU/256 GB memory)

DN: 25 GB (96-core CPU/768 GB memory); 20 GB (72-core CPU/576 GB memory); 15 GB (64-core CPU/512 GB memory); 5 GB (32-core CPU/256 GB memory)

Setting suggestions:

A larger value indicates that the optimizer prefers index scanning, and a smaller value indicates that the optimizer prefers full table scanning. Generally, the value is half of the value of shared_buffers. More radically, you can set the value to three fourth of the value of shared_buffers.

allocate_mem_cost

Parameter description: Specifies the query optimizer's estimated cost of creating a hash table for memory space using hash join. This parameter is used for optimization when the hash join estimation is inaccurate.

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

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

Default value: 0

Parent topic: Query Planning