How to Avoid Multiple Function Evaluations with PostgreSQL's `(func()).*` Syntax?

Optimizing PostgreSQL Queries: Avoiding Redundant Function Calls with (func()).*

Using the (func()).* syntax in PostgreSQL to access results from a function returning a composite type or TABLE can lead to performance issues due to repeated function evaluations. This occurs because the syntax expands into multiple column references, each triggering a separate function call. The number of calls equals the number of columns in the result set.

The Problem: Inefficient Multiple Function Calls

The (func()).* approach is inefficient because it doesn't optimize for minimal function calls. Ideally, the function should be called only once to retrieve the entire result set.

Solutions:

PostgreSQL 9.3 and later offer a straightforward solution using LATERAL joins:

<code class="language-sql">SELECT mf.*
FROM some_table
LEFT JOIN LATERAL my_func(some_table.x) AS mf ON true;</code>

LATERAL ensures that my_func is executed only once per row in some_table, significantly improving performance.

For older PostgreSQL versions (prior to 9.3), workarounds are necessary:

Workaround 1: OFFSET 0 Hack

This method leverages the OFFSET 0 clause to force a single function evaluation:

<code class="language-sql">SELECT (mf).* FROM (
    SELECT my_func(x) AS mf FROM some_table OFFSET 0
) sub;</code>

Workaround 2: Common Table Expression (CTE)

A CTE provides a cleaner alternative:

<code class="language-sql">WITH tmp AS (
    SELECT my_func(x) FROM some_table
)
SELECT (mf).* FROM tmp;</code>

Both workarounds ensure that my_func is called only once, retrieving the complete result set before accessing individual columns.

Root Cause: Parser Expansion

The multiple evaluations arise from how PostgreSQL's parser interprets (func()).*. It translates this into separate column accesses, resulting in multiple function invocations. This limitation has been addressed in newer versions with the introduction of LATERAL joins.

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