How Can I Efficiently Retrieve the Total Result Count for Paginated Data in PostgreSQL?

Optimize retrieval of paging data result count

When implementing the paging function of data retrieval, determining the total number of pages is crucial for accurately rendering the paging control. A common approach is to run two queries: one using the COUNT() function to get the total number of results, and another using the LIMIT clause to display the current page data.

However, this approach is inefficient. PostgreSQL has introduced window functions since version 8.4, providing a better solution.

Use window functions

The window function COUNT(*) OVER() allows us to calculate the total number of results in a query while obtaining limited data. An example is as follows:

<code class="language-sql">SELECT foo, COUNT(*) OVER() AS full_count
FROM bar
WHERE <some condition="">
ORDER BY <some col="">
LIMIT <pagesize>
OFFSET <offset>;</code>

The full_count column provides the total number of results before the LIMIT and OFFSET clauses are applied. Note that this approach may impact performance as it requires counting all qualifying rows. The impact is minimal for smaller tables or where full_count is less than OFFSET LIMIT. However, for larger result sets, it's worth considering alternatives.

Alternative method for final count

Besides window functions, there are other ways to retrieve the final count without calculating the full count:

• PostgreSQL internal logging: PostgreSQL maintains information about the number of rows affected by the last SQL command. Clients can access this data, for example, using GET DIAGNOSTICS in plpgsql, or pg_num_rows in PHP.
• SQL commands with LIMIT/OFFSET: Execute a separate query using the same LIMIT and OFFSET values ​​as the original query. The final count can be obtained from the number of rows in the result set.

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