How to diagnose and fix the 42P18 indeterminate_datatype error code in Postgres.

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The 42P18 indeterminate_datatype error in PostgreSQL occurs when the database engine cannot determine the data type of a parameter or a column in a query. This can happen in various scenarios, such as when using prepared statements, functions, or when performing operations that require the database to infer data types.

Here are several examples of scenarios that can cause this error and how to diagnose and fix them:

  1. Using Parameters in Prepared Statements
    When using prepared statements, you might not provide enough context for PostgreSQL to infer the data type of a parameter. Example:
   PREPARE myplan AS SELECT * FROM my_table WHERE column = $1;
   EXECUTE myplan(NULL);

This might cause an indeterminate datatype error because PostgreSQL doesn’t know the type of $1.

Fix:
You can explicitly cast the parameter to the correct data type.

   EXECUTE myplan(CAST(NULL AS desired_type));

Replace desired_type with the actual data type of the column you’re comparing against.

  1. Using Functions Without Clear Data Types
    Custom functions that do not have a clearly defined return type or input parameters can also cause this error. Example:
   CREATE FUNCTION my_function(parameter) RETURNS SETOF record AS $$
   ...
   $$ LANGUAGE sql;

If you call this function without specifying the data type of parameter, you might encounter the indeterminate datatype error.

Fix:
Define the function with explicit data types for all parameters and the return type.

   CREATE FUNCTION my_function(parameter desired_type) RETURNS SETOF return_type AS $$
   ...
   $$ LANGUAGE sql;

Replace desired_type and return_type with the specific data types.

  1. Inserting or Selecting Into Arrays Without Type
    When dealing with arrays, if you do not specify the data type, PostgreSQL might not be able to infer it. Example:
   SELECT ARRAY(SELECT column FROM my_table WHERE condition);

If condition is ambiguous, PostgreSQL may not know the data type of column.

Fix:
You can explicitly cast the column to the correct data type.

   SELECT ARRAY(SELECT column::desired_type FROM my_table WHERE condition);

Replace desired_type with the actual data type of column.

  1. Using Arbitrary Values in Queries
    Arbitrary values in queries without explicit casting can lead to indeterminate datatype errors. Example:
   SELECT * FROM my_table WHERE column IN (value1, value2, value3);

If the values are not of the same data type as column, the error may occur.

Fix:
Explicitly cast the values to the data type of the column.

   SELECT * FROM my_table WHERE column IN (CAST(value1 AS desired_type), CAST(value2 AS desired_type), CAST(value3 AS desired_type));

Replace desired_type with the actual data type of column.

To diagnose the 42P18 indeterminate_datatype error, you should examine your SQL query to ensure that all literals, parameters, and function calls have explicit and matching data types. If you use tools or ORMs, make sure they are correctly configured to handle data types.

For further guidance, you can refer to the PostgreSQL documentation on error codes and consult community discussions and solutions, such as those found on Stack Overflow or GitHub issues related to this error.

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