Default Behavior: Many Cross-Type Comparisons Work in PostgreSQL

For standard column types, PostgreSQL will often implicitly cast string literals to match the column's data type when comparing, so queries like the following work out of the box:

-- If user_id is integer
SELECT * FROM users WHERE user_id = '123';  -- PostgreSQL casts '123' to integer

As long as the string literal can be successfully cast to the type of the column, the comparison will succeed without explicit modification or custom operator creation.

However:

• If the literal can't be cast (e.g. 'abc' to integer), you’ll get a runtime error.
• Explicit casts (col::text = 'someval') are only required in ambiguous or complex cases, or for user-defined types.

Special Challenges: Large Applications and Massive Codebases

For enterprise-scale applications, involving numerous tables and thousands of queries and procedures:

• Manually adding explicit casts throughout the codebase is often impractical or impossible.
• Refactoring risks introducing subtle bugs.
• Migration timelines are extended, impacting business agility.
• Legacy code ownership or third-party dependencies may limit scope for code changes.

In such contexts, teams desire a solution that minimizes application-level code edits while achieving full PostgreSQL compatibility.

A Practical PostgreSQL Solution: Custom Comparison Operators

To facilitate migration with minimal query changes, you can create PostgreSQL functions and custom equality operators to extend the = operator, allowing seamless cross-type equality comparisons.

How Custom Operators Work

1. Create Conversion Functions

Define functions that accept two differing types, cast internally, and return a boolean reflecting equality. Example for text and integer:

CREATE OR REPLACE FUNCTION public.text_eq_integer(text, integer) 
RETURNS boolean AS $$
    SELECT $1 = $2::text;
$$ LANGUAGE SQL IMMUTABLE STRICT;

CREATE OR REPLACE FUNCTION public.integer_eq_text(integer, text) 
RETURNS boolean AS $$
    SELECT $1::text = $2;
$$ LANGUAGE SQL IMMUTABLE STRICT;

2. Define Equality Operators Referencing These Functions

Register new = operators that PostgreSQL invokes when comparing these types:

CREATE OPERATOR public.= (
    LEFTARG = text,
    RIGHTARG = integer,
    PROCEDURE = public.text_eq_integer,
    COMMUTATOR = =
);

CREATE OPERATOR public.= (
    LEFTARG = integer,
    RIGHTARG = text,
    PROCEDURE = public.integer_eq_text,
    COMMUTATOR = =
);

3. Configure Commutator Links

The COMMUTATOR = attribute ensures that the equality operator is symmetric; PostgreSQL understands a = b is equivalent to b = a.

Why This Solution Matters for Large Migrations

• Avoid rewriting millions of queries or adding explicit casts manually.
• Preserve existing business logic and query semantics without intrusive changes.
• Accelerate migration timelines and reduce costs.
• Allow incremental code refactoring when convenient.
• Maintain reliable, consistent enforcement of equality checks at the database level.

Important Considerations

• Invalid casts cause runtime errors: Query literals that cannot be cast to the target type will fail.
• Custom operators do not automatically optimize index usage; additional operator class work may be required for performance.
• Overriding core operators increases the complexity of maintenance and requires robust documentation.
• Extensive testing is essential to ensure logical correctness and performance post-migration.

Summary

PostgreSQL’s implicit casting handles most cross-type equality comparisons well, which means custom operators are only needed where implicit casts do not suffice or legacy SQL Server behavior must be exactly preserved.

Creating custom equality functions and operators is a practical tool for easing large, legacy SQL Server migrations to PostgreSQL and minimizing disruptive query rewrites.