How to diagnose and fix the 22038 singleton_sql_json_item_required error code in Postgres.

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The 22038 error code in PostgreSQL, which stands for singleton_sql_json_item_required, indicates that a single JSON item was required but the JSON value contained multiple items or was not a singleton. This error typically occurs in the context of SQL/JSON functions and operators that expect a single JSON item as input.

Here are some examples and sample code to diagnose and fix this error:

Example 1: Using JSON Functions That Require a Single Item

When using functions like jsonb_populate_record or jsonb_to_record, which expect a single JSON object, passing an array or multiple objects can cause this error.

Problematic Query:

SELECT jsonb_populate_record(null::my_table_type, '[{"key": "value1"}, {"key": "value2"}]');

Sample Code to Fix:
Ensure that you are passing a single JSON object to the function.

SELECT jsonb_populate_record(null::my_table_type, '{"key": "value1"}');

Example 2: Extracting a JSON Value That Is Not Singleton

When you attempt to extract a JSON value from a path that leads to multiple items, you might encounter this error.

Problematic Query:

SELECT jsonb_path_query_first(my_json_column, '$.key'::jsonpath) FROM my_table;

Assuming my_json_column contains multiple items for the key ‘key’.

Sample Code to Fix:
Make sure that the JSON path leads to a single item.

SELECT jsonb_path_query_first(my_json_column, '$.key[0]'::jsonpath) FROM my_table;

Example 3: Using JSON Path Expressions That Return Multiple Items

When using JSON path expressions that return multiple items in a context where a single item is required, you will encounter this error.

Problematic Query:

SELECT jsonb_path_query_first(my_json_column, '$.key.*'::jsonpath) FROM my_table;

Assuming my_json_column contains an object with multiple values for ‘key’.

Sample Code to Fix:
Adjust the JSON path to target a specific item.

SELECT jsonb_path_query_first(my_json_column, '$.key.value1'::jsonpath) FROM my_table;

To diagnose the 22038 error, inspect the JSON data and the queries to confirm that a single JSON item is being provided where required. If the operation expects a singleton (a single value), you must adjust your JSON path expression or the data structure to ensure that only one item is passed to the function or operator.

For a guide on how to resolve PostgreSQL Error Code 22038, you can refer to resources like USA VPS, which provide step-by-step instructions.

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