How to diagnose and fix the HV010 fdw_function_sequence_error error code in Postgres. 

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The HV010 error code in PostgreSQL corresponds to an fdw_function_sequence_error, which is related to foreign data wrappers (FDWs). This error typically occurs when there is an issue with the sequence of functions being called on a foreign data wrapper. For instance, it can happen if a function that requires a prior action to be called (like a transaction or connection setup) is invoked without the necessary preliminary steps.

To diagnose and fix an HV010 error, you need to ensure that you are following the correct sequence of function calls as required by the foreign data wrapper’s API you are using. Here are some steps and examples to help you understand and resolve the issue:

  1. Review the FDW documentation: Each foreign data wrapper can have its own sequence of operations. Make sure you understand the required sequence for the FDW you are using by reviewing its documentation.
  2. Check the transaction sequence: Ensure that the transaction sequence is correct. For example, you shouldn’t be attempting to fetch data before establishing a connection to the foreign server.
  3. Examine the code for correct function order: Look for any function calls that might be out of order. For example, you might be trying to execute a query before a transaction has been properly started.
  4. Ensure proper error handling: Implement error handling to catch and log any exceptions or errors that might indicate a sequence error, which can help identify where the sequence is going wrong.

Here is an example of what correct sequence code might look like in Python using Psycopg, a PostgreSQL adapter:

import psycopg2
from psycopg2 import sql

# Connect to your PostgreSQL database
conn = psycopg2.connect("dbname=test user=postgres")

# Create a cursor object
cur = conn.cursor()

# Start a transaction
cur.execute("BEGIN;")

# Set up the foreign data wrapper connection
cur.execute("CREATE SERVER IF NOT EXISTS my_foreign_server FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host 'foreign_host', dbname 'foreign_db');")

# Create a user mapping
cur.execute("CREATE USER MAPPING FOR CURRENT_USER SERVER my_foreign_server OPTIONS (user 'foreign_user', password 'foreign_pass');")

# Import a foreign schema
cur.execute("IMPORT FOREIGN SCHEMA public LIMIT TO (table1, table2) FROM SERVER my_foreign_server INTO public;")

# Commit the transaction
cur.execute("COMMIT;")

# Now you can use the foreign tables as if they were local
cur.execute("SELECT * FROM table1;")
rows = cur.fetchall()
for row in rows:
    print(row)

# Close the cursor and connection
cur.close()
conn.close()

In this example, the sequence of operations is important: you begin a transaction, set up the FDW server and user mapping, import the foreign schema, and then commit the transaction before querying the foreign table.

If you’re encountering the HV010 error, review the sequence of these operations in your own code. Ensure that each step is being executed in the correct order and that all necessary preliminary steps are completed before moving on to subsequent operations.

Remember that the exact sequence and operations may vary depending on the foreign data wrapper you are using, so it’s crucial to consult the specific FDW documentation for guidance.

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