Why Are My Function's Returned Columns Concatenated in My PostgreSQL Query?

Record Returned from Function Shows Concatenated Columns: Unraveling the Cause

In a query, joining tables and retrieving historical data from a function, users may encounter an issue where columns returned from the function appear concatenated into a single string. This anomaly arises when using the function directly within the main query.

Understanding the Issue

When querying a function that returns multiple columns, such as:

it returns a record containing the requested data. However, when this record is embedded in another query, PostgreSQL interprets it as a single concatenated string.

Decomposing Function Output

To remedy this, users should extract the individual columns from the function's record. In PostgreSQL versions 9.3 and later, this can be achieved using the JOIN LATERAL syntax, which allows seamless integration of functions into the FROM clause:

The ON TRUE clause ensures that all rows from the left-hand table are joined with the result of the function, regardless of the number of rows returned.

In PostgreSQL 9.2 and earlier, extracting column data requires an explicit subquery:

Here, the subquery assigns the result of the function call to a record named rec. The outer query can then refer to the record's columns, such as a.rec.timestamp.

Avoiding Not In (Subquery)

Lastly, it's recommended to avoid using NOT IN (subquery) for set exclusion. Instead, prefer the NOT EXISTS clause, which is more efficient and less prone to performance issues.

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