How to diagnose and fix the 42P03 duplicate_cursor error code in Postgres. 

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The 42P03 error code in PostgreSQL indicates a “duplicate_cursor” error. This error occurs when you attempt to declare a new cursor with a name that is already used by an existing cursor within the same transaction block. Cursors in PostgreSQL are like iterators that are used to retrieve a few rows at a time from the result set of a larger query.

To diagnose and fix this error, you need to ensure that each cursor has a unique name within a given transaction. Here are some scenarios where this error might occur, along with examples and sample code to diagnose and fix the issue:

Scenario 1: Re-declaring an Existing Cursor

If you declare a cursor and then declare another cursor with the same name within the same transaction, you will encounter a duplicate_cursor error.

Example:

BEGIN;

DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;

-- Attempting to declare another cursor with the same name.
DECLARE my_cursor CURSOR FOR SELECT * FROM another_table;

END;

Fix:
Rename one of the cursors to ensure each has a unique name:

BEGIN;

DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;

-- Renamed the second cursor to avoid duplication.
DECLARE another_cursor CURSOR FOR SELECT * FROM another_table;

END;

Scenario 2: Using the Same Cursor Name in Different Functions or DO Blocks

Even if cursors are declared in different functions or anonymous code blocks (DO statements), if the functions or blocks are called within the same transaction, cursor names must still be unique.

Example:

CREATE FUNCTION my_function() RETURNS void AS $$
BEGIN
  DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;
  -- Function logic...
END;
$$ LANGUAGE plpgsql;

BEGIN;

-- This will work fine if called in separate transactions.
SELECT my_function();

-- But re-declaring the cursor here will cause an error.
DECLARE my_cursor CURSOR FOR SELECT * FROM another_table;

END;

Fix:
Ensure cursor names are unique across all functions and code blocks within the same transaction:

-- Adjust the function or code blocks to use different cursor names.
CREATE FUNCTION my_function() RETURNS void AS $$
BEGIN
  DECLARE func_cursor CURSOR FOR SELECT * FROM my_table;
  -- Function logic...
END;
$$ LANGUAGE plpgsql;

BEGIN;

SELECT my_function();

-- Now this declaration won't cause a conflict.
DECLARE another_cursor CURSOR FOR SELECT * FROM another_table;

END;

Scenario 3: Reusing a Cursor Name After Closing It

You might mistakenly assume that after closing a cursor, its name can be immediately reused within the same transaction. However, the name remains reserved until the transaction is completed.

Example:

BEGIN;

DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;
-- Some operations using the cursor...
CLOSE my_cursor;

-- Attempting to redeclare the cursor with the same name, thinking it's now free.
DECLARE my_cursor CURSOR FOR SELECT * FROM another_table;

END;

Fix:
Complete the transaction before reusing the cursor name, or use a different name:

BEGIN;

DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;
-- Some operations using the cursor...
CLOSE my_cursor;

COMMIT; -- End the current transaction.

BEGIN; -- Start a new transaction if necessary.

-- Now it's safe to use the same cursor name.
DECLARE my_cursor CURSOR FOR SELECT * FROM another_table;

END;

General Tips for Diagnosis and Fixes:

  1. Ensure Unique Cursor Names:
    Within a transaction, make sure each cursor has a unique name.
  2. Use Naming Conventions:
    Adopt a naming convention for cursors that reduces the risk of name collisions, such as including function names or other context in the cursor name.
  3. Review Code for Cursor Declarations:
    Search your code for all instances of cursor declarations to ensure there are no duplicates within the same transaction scope.
  4. Close and Complete Transactions:
    Remember that closing a cursor does not free its name until the transaction is completed. Be mindful of transaction boundaries when reusing cursor names.

By following these guidelines, you can avoid the 42P03 duplicate_cursor error and ensure that your PostgreSQL transactions run smoothly. For more information on handling cursors in PostgreSQL, you can refer to the official documentation.

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