How Can LATERAL JOIN Optimize Multiple Set-Returning Function Calls with Array Arguments in PostgreSQL?

Using LATERAL JOIN to Handle Multiple Function Calls with Array Arguments

In PostgreSQL, handling multiple calls to a set-returning function with an array argument can be challenging. This becomes particularly apparent when the function operates on an array of rows and returns a set of rows with an additional column.

Understanding the Limitation

The standard approach involves passing the array as a single parameter to the function and relying on a subsequent query to extract the relevant data. However, this approach often leads to multiple calls to the function, with one call for each row in the array.

The LATERAL JOIN Solution

To address this limitation, PostgreSQL 9.3 and later introduced the LATERAL JOIN operator. This operator allows for joining tables on a condition evaluated row-by-row, thereby eliminating the need for multiple function calls.

The following query demonstrates the use of LATERAL JOIN to join a set of functions:

SELECT sub.dataid, f.*
FROM  (
   SELECT dataid, array_agg(data) AS arr
   FROM   dataset
   WHERE  dataid = something
   GROUP  BY 1
   ) sub
LEFT   JOIN LATERAL foo(sub.arr) f ON true;

The subquery creates an array of data for each dataid and joins it with the foo function using LATERAL JOIN. This ensures that the function is called only once, and the result is joined with the subquery data.

Optimizing the Query

If the foo() function can return no rows, using LEFT JOIN LATERAL ... ON true preserves all rows to the left of the join. Otherwise, consider using CROSS JOIN LATERAL foo(sub.arr) or the shorthand , foo(sub.arr) to exclude rows without results.

This approach not only avoids multiple function calls but also improves performance and simplifies queries, especially when working with large datasets.

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