In PostgreSQL, query optimization plays a crucial role in achieving efficient query performance. Cost estimation is a key component of this optimization process, allowing the database to estimate the cost of various operations. This article focuses on the estimation of the sequential scan cost, one of the fundamental operations in PostgreSQL, and explores how it is calculated for a specific query.
Understanding Sequential Scan Cost:
The cost of a sequential scan operation is estimated using the cost_seqscan() function. To illustrate the estimation process, let's consider the following query:
testdb=# SELECT * FROM tbl WHERE id < 8000;
Estimating Run Cost:
The run cost of a sequential scan is composed of two parts: the CPU run cost and the disk run cost. The CPU run cost is calculated based on the number of tuples (Ntuple) and the CPU costs (cpu_tuple_cost and cpu_operator_cost) set in the postgresql.conf file. Similarly, the disk run cost is determined by the number of pages (Npage) and the sequential page cost (seq_page_cost). The default values for these costs are 1.0, 0.01, 0.0025, and 1.0, respectively.
To estimate the run cost, we can use the following equation:
'run cost' = ('cpu run cost' + 'disk run cost') = ((cpu_tuple_cost + cpu_operator_cost) × Ntuple) + (seq_page_cost × Npage)
To determine Ntuple and Npage, we can execute the following query:
testdb=# SELECT relpages, reltuples FROM pg_class WHERE relname = 'tbl';
In our example, Ntuple is 10,000 and Npage is 45.
Plugging these values into the equation, we get:
'run cost' = ((0.01 + 0.0025) × 10,000) + (1.0 × 45) = 170.0
Calculating Total Cost:
The total cost is the sum of the start-up cost and the run cost. In the case of a sequential scan, the start-up cost is 0.0. Therefore, the total cost is equal to the run cost. In our example, the total cost is 170.0.
Confirming the Cost Estimation:
To validate the cost estimation, we can use the EXPLAIN command on our query:
testdb=# EXPLAIN SELECT * FROM tbl WHERE id < 8000;
The output will display the query plan with the associated costs. In our case, we can observe that the start-up cost is 0.00 and the total cost is 170.00. Additionally, it is estimated that 8,000 rows (tuples) will be selected by scanning all rows.
Understanding how PostgreSQL estimates the cost of a sequential scan operation is essential for query optimization. By analyzing the CPU and disk costs based on the number of tuples and pages, PostgreSQL can determine the most efficient execution plan. By considering the estimated costs and analyzing the query plan, developers and database administrators can make informed decisions to optimize performance in PostgreSQL.
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