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Description

This error indicates that PostgreSQL encountered an attempt to convert data between incompatible types or perform an invalid cast operation. It typically arises when an implicit cast is not supported, or an explicit cast operation is syntactically incorrect or semantically impossible.

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Error Message

cannot coerce

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Known Causes

4 known causes

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Implicit Data Type Mismatch

This occurs when PostgreSQL tries to automatically convert data between two incompatible types, such as comparing a string with a numeric value without proper casting.

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Invalid Explicit Cast

An explicit `CAST` operation was attempted using incorrect syntax or specifying a conversion between types that PostgreSQL does not support.

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Function Argument Mismatch

A function or operator was called with arguments of data types that do not match its expected parameters, and no valid implicit cast could be applied.

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Data Insertion Type Conflict

Attempting to insert or update data into a table column where the provided value's data type is incompatible with the column's defined type, without a valid implicit or explicit cast.

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Solutions

4 solutions available

1. Explicitly Cast Data Types easy

Manually convert values to the expected data type before insertion or comparison.

1

Identify the columns or expressions involved in the conversion error. This often occurs in `INSERT`, `UPDATE`, `SELECT` statements with `WHERE` clauses, or when joining tables.

2

Use the `::` cast operator or the `CAST()` function to explicitly convert the data to the target type. For example, if you're trying to insert a string that looks like a number into an integer column, cast it first.

INSERT INTO my_table (numeric_column) VALUES ('123'::integer);

3

Alternatively, use the `CAST()` function:

INSERT INTO my_table (numeric_column) VALUES (CAST('123' AS integer));

4

Apply this casting to all problematic values in your queries.

2. Correct Data Type in Table Definition medium

Alter the table schema to use the appropriate data type for the column.

1

Determine the correct data type for the column based on the data you intend to store. For example, if a column is defined as `VARCHAR` but should store dates, it should be `DATE`.

2

Use the `ALTER TABLE` statement to change the data type of the column. This may involve data loss or errors if existing data cannot be converted.

ALTER TABLE my_table ALTER COLUMN my_column TYPE new_data_type USING my_column::new_data_type;

3

If existing data is problematic, you might need to clean it up or transform it before altering the type. For instance, if a `numeric_column` contains non-numeric strings, you might need to update those rows first.

UPDATE my_table SET my_column = '0' WHERE my_column ~ '[^0-9]';
ALTER TABLE my_table ALTER COLUMN my_column TYPE integer USING my_column::integer;

3. Normalize Data Before Insertion medium

Clean and validate data in your application code before sending it to PostgreSQL.

1

Before executing `INSERT` or `UPDATE` statements, review the data being sent from your application.

2

Implement validation logic in your application to ensure that the data conforms to the expected data types of your PostgreSQL columns. For example, if a column expects an integer, ensure the input is a valid integer string or number.

3

Perform necessary transformations. For instance, if a date is in an unexpected format, reformat it to the ISO 8601 standard (`YYYY-MM-DD`) which PostgreSQL understands well.

4

Handle potential nulls or empty strings appropriately, ensuring they are compatible with the target column's nullability and data type.

4. Review Join Conditions and Functions medium

Ensure data types match in join conditions and function arguments.

1

Examine `JOIN` clauses to ensure that the columns being joined have compatible data types. If they don't, use explicit casting on one or both sides of the join.

SELECT t1.* FROM table1 t1 JOIN table2 t2 ON t1.id::text = t2.uuid_as_text;

2

Check the arguments passed to built-in PostgreSQL functions. Many functions have specific data type requirements.

SELECT to_char(my_date_column, 'YYYY-MM-DD') FROM my_table; -- Correct
SELECT to_char(my_numeric_column, 'FM999') FROM my_table; -- Incorrect if my_numeric_column is not text-like

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If you are using custom functions, verify that the data types of the arguments passed to them match the declared parameter types of the function.

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Related Errors

5 related errors

PostgreSQL Error 42846: Invalid Data Type Conversion