transform_null_equals — PostgreSQL configuration parameter

When on, expressions of the form expr = NULL (or NULL = expr) are treated as expr IS NULL, that is, they return true if expr evaluates to the null value, and false otherwise.

At a glance

What it does

When on, expressions of the form expr = NULL (or NULL = expr) are treated as expr IS NULL, that is, they return true if expr evaluates to the null value, and false otherwise. The correct SQL-spec-compliant behavior of expr = NULL is to always return null (unknown). Therefore this parameter defaults to off.

However, filtered forms in Microsoft Access generate queries that appear to use expr = NULL to test for null values, so if you use that interface to access the database you might want to turn this option on. Since expressions of the form expr = NULL always return the null value (using the SQL standard interpretation), they are not very useful and do not appear often in normal applications so this option does little harm in practice. But new users are frequently confused about the semantics of expressions involving null values, so this option is off by default.

(Description quoted from the official PostgreSQL documentation.)

How to apply a change

Can be set per session with SET, per role/database with ALTER ROLE/DATABASE ... SET, or globally in postgresql.conf.

Inspect the current value and source with SHOW transform_null_equals; or SELECT name, setting, unit, context, source FROM pg_settings WHERE name = 'transform_null_equals';.

Tuning guidance

This controls backward-compatibility behaviour, not performance. Keep it at the modern default unless a specific legacy application depends on the older behaviour; turning compatibility flags on to paper over application bugs stores up problems for a future upgrade. Treat any non-default value as technical debt to remove.

Reference

PostgreSQL documentation — transform_null_equals.