How Can I Efficiently Retrieve the Total Result Count Before Applying a LIMIT Clause in PostgreSQL?

Efficiently obtain the result count before LIMIT

When working with database data, determining the total number of pages is crucial for rendering paginated navigation controls. The traditional approach is to perform the query twice, but this is inefficient. This article explores alternative methods of determining result counts while minimizing the number of queries required.

PostgreSQL window functions

Starting with PostgreSQL version 8.4, window functions provide a powerful way to collect both full count results and restricted result sets in a single query. Using the OVER() clause in conjunction with the count(*) function, developers can calculate the total number of records that will be returned before applying the LIMIT clause.

SELECT foo,
     count(*) OVER() AS full_count
FROM   bar
WHERE  <some condition="">
ORDER  BY <some col="">
LIMIT  <pagesize>
OFFSET <offset>;

It is important to note that this approach may be more computationally expensive than simply retrieving a restricted result set. Therefore, it is recommended to use this method when both a complete count and a restricted result set are required.

Alternative way to get count

If a full count is not required, there are other ways to retrieve the count of affected rows. PostgreSQL maintains internal bookkeeping information that is accessible to clients. In PHP, you can use the pg_num_rows function for this purpose. Additionally, plpgsql provides the GET DIAGNOSTICS command to retrieve row counts.

Execution order of SELECT query

It is important to understand the order in which SELECT queries are executed, which affects how the LIMIT and OFFSET clauses are applied. The WHERE clause filters qualifying rows from the base table, and the window function is applied to the filtered subset.

OFFSET performance considerations

Although OFFSET is useful for paging, it becomes less and less efficient as the number of rows in the table grows. Please consider using alternatives such as leveraging cursors or using OFFSET to optimize queries.

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