How Can I Efficiently Determine the Total Row Count Before Applying LIMIT in PostgreSQL Queries?

Efficiently obtain the total number of rows before LIMIT in PostgreSQL query

Database data paging usually requires determining the total number of pages to render the paging control. Typically, this requires running two separate queries: one using COUNT() to get the total, and another using LIMIT to retrieve the current page's data.

This method is inefficient. Fortunately, there is a better way in PostgreSQL to get the total before applying LIMIT: use window functions.

SQL window functions

Window functions introduced in PostgreSQL 8.4 allow us to perform calculations on a data set defined by a "window". By specifying a suitable window, we can retrieve the total without affecting the LIMIT operation.

Consider the following query:

<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>

Here, full_count provides the total number of rows before LIMIT is applied.

Note: Using window functions in this way can be computationally more expensive than the traditional two-query approach. This is because all rows must be counted regardless of paging parameters.

Alternative method for total number

In some cases there is no need to get the total before LIMIT. An alternative is:

1. Run the query using LIMIT and OFFSET.
2. Use a client function to count the number of affected rows (e.g. GET DIAGNOSTICS in PL/pgSQL or pg_num_rows in PHP).

Other considerations

1. The order of events in SQL queries affects performance. LIMIT and OFFSET operations are applied after filtering, grouping, and other operations.
2. Using LIMIT and OFFSET on large tables is inefficient. Consider using other methods for better performance.

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