How to Efficiently Merge Columns in PostgreSQL?

Merging Columns for a New Column in PostgreSQL

In PostgreSQL, the task of combining two existing columns into a new one arises frequently. While the familiar concat() function is a common approach, let's explore other alternatives and when they excel.

Concatenation with ||

If you're certain that your columns will never contain null values, the traditional concatenation operator || remains the most efficient choice. Simply write:

SELECT col_a || col_b;

concat() for Handling Null Values

However, when null values are a possibility, concat() shines. Unlike ||, concat() guarantees a non-null result, even when all its arguments are null. Its syntax:

SELECT concat(col_a, col_b);

COALESCE for Ensuring Non-Empty Values

If null values can disrupt your desired output, consider using COALESCE in combination with ||:

SELECT COALESCE(col_a, '') || COALESCE(col_b, '');

This approach ensures that the result is an empty string if either column is null.

concat() for Complex Cases

When dealing with multiple null columns or complex expressions, concat() proves particularly useful. For example:

SELECT CASE WHEN (col_a, col_b) IS NULL THEN NULL
            ELSE concat(col_a, col_b) END;

concat_ws() for Adding Separators

For situations where you need to insert separators between merged elements, concat_ws() offers a convenient option:

SELECT concat_ws(' - ', col_a, col_b, col_c);

Stability Considerations

Note that while concat() and concat_ws() are generally stable functions, they can potentially trigger datatype output functions that rely on locale settings. This can affect their direct usage in index expressions.

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