PostgreSQL Error 42846 cannot_coerce – causes and fixes

What is PostgreSQL error 42846 cannot_coerce?

PostgreSQL raises error 42846 when it cannot automatically convert, or coerce, one data type to another in a SQL expression. The server stops the query because executing it could lose data or break type safety.

The error is common during INSERT, UPDATE, and SELECT statements that mix incompatible types or omit explicit casts.

Fixing it quickly keeps queries reliable and prevents silent truncation or precision loss.

What Causes This Error?

Explicitly assigning a value of one data type to a column of another incompatible type triggers the error. PostgreSQL only supports implicit casts between safe, predefined pairs of types.

Joining tables on differently typed columns or comparing mismatched literals can also fail.

Functions that expect a specific type but receive another unsupported type will raise the same code.

How to Fix PostgreSQL Error 42846

Add an explicit cast with the :: operator or CAST() function so PostgreSQL knows how to convert the value. Alternatively, alter the table to use a compatible type when design changes permit.

Check operator precedence.

Enclose casted values in parentheses to ensure they run before comparison or arithmetic operations.

Common Scenarios and Solutions

Attempting to insert text '123' into an integer column fails unless you cast it: INSERT INTO t(i) VALUES ('123'::int);

Joining varchar id to bigint id requires casting one side: ... ON a.id::bigint = b.id.

Aligning both columns to the same type in the schema is safer long term.

Best Practices to Avoid This Error

Design schemas with consistent data types for primary-foreign key pairs. Document column types so callers supply the correct literal or parameter type.

Enable strict typing in ORMs and use Galaxy’s AI copilot to review generated SQL.

The editor highlights type mismatches before you run them, preventing runtime failures.

Related Errors and Solutions

Error 42804 (datatype_mismatch) signals a similar conflict but in assignment contexts. Error 22P02 (invalid_text_representation) appears when a cast fails because the literal format is wrong. The resolution pattern is the same: cast correctly or change types.

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