Understanding Cost Estimation in Single-Table Queries in PostgreSQL

In PostgreSQL, query optimisation plays a vital role in achieving optimal query performance. This article explores how PostgreSQL estimates costs in single-table queries and provides insights into the different types of costs and their significance in query optimisation.
Estimating Costs: Functions and Dimensions:
Cost estimation in PostgreSQL is based on relative performance indicators rather than absolute values. The functions defined in costsize.c are responsible for estimating costs for different operations executed by the executor. For instance, the cost_seqscan() and cost_index() functions estimate the costs of sequential scans and index scans, respectively.
Types of Costs: Start-up, Run, and Total:
PostgreSQL distinguishes three types of costs: start-up, run, and total. The start-up cost represents the cost incurred before the first tuple is fetched, such as reading index pages. The run cost, on the other hand, reflects the cost of fetching all tuples. The total cost is the sum of the start-up and run costs. It is important to note that the start-up and run costs are estimated independently, and the total cost is derived from their combination.
Understanding EXPLAIN and Cost Display:
The EXPLAIN command in PostgreSQL provides insights into the cost estimation process. It displays both the start-up and total costs for each operation in a query plan. For instance, when executing a simple single-table query, the EXPLAIN output includes information about the sequential scan operation, with the cost section displaying the start-up and total costs.
Exploring Specific Operations: Sequential Scan, Index Scan, and Sort:
In this article, we delve into the estimation process for three specific operations: sequential scan, index scan, and sort. By examining these operations in detail, we gain a deeper understanding of how PostgreSQL estimates their costs during query optimization.
Practical Example:
To illustrate the cost estimation process, we use a specific table and index. We create a table called tbl with columns id and data, and an index called tbl_data_idx on the data column. We then populate the table with data and analyze it to generate statistics for accurate cost estimation. The article presents a practical example using the created table and index to showcase the cost estimation process in action.
Cost estimation is a crucial aspect of query optimization in PostgreSQL. By estimating costs for various operations, PostgreSQL's optimizer can make informed decisions about the most efficient execution plan for a query. Understanding the different types of costs and their estimation methods empowers developers and database administrators to optimize query performance and ensure efficient utilization of resources in single-table queries within PostgreSQL.