How to Prevent Multiple Function Calls with `(func()).*` in Older PostgreSQL Versions?

Avoiding Repeated Function Calls with (func()).* in Older PostgreSQL Versions

The Issue

In PostgreSQL versions prior to 9.3, using the (func()).* syntax (where func returns a table) can lead to unintended multiple executions of the function. This significantly impacts query performance.

Solutions

Several methods effectively circumvent this problem:

1. Subquery Approach:

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

2. OFFSET 0 Technique:

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

3. CTE (Common Table Expression) Approach:

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

4. LATERAL JOIN (PostgreSQL 9.3 and later):

<code class="language-sql">
SELECT mf.*
FROM some_table
LEFT JOIN LATERAL my_func(some_table.x) AS mf ON true;
```  This is the preferred method for PostgreSQL 9.3 and above.


### Explanation

The root cause lies in how PostgreSQL's parser handles `(func()).*` in older versions.  The wildcard expands into individual column selections, mistakenly causing the function to be called repeatedly for each column.


### Why Repeated Calls Occur

Pre-9.3 PostgreSQL parsers interpret `(func()).*` by replicating nodes within the parse tree.  This replication results in a separate function call for every selected column, even if a single call would suffice.</code>

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