How to skip blocked fields in the database when using the SELECT operator?

When using SELECT, if a field is locked for a transaction or user, attempting to read it usually results in an error or blocking until the lock is released, rather than skipping it. Skipping locked fields at the SQL SELECT level is not a standard behavior or feature. Instead, transaction and lock management mechanisms at the database level are used.

Possible approaches to handle such a situation:

1. Transaction management: Use transaction isolation levels (for example, READ COMMITTED can avoid reading uncommitted data, while READ UNCOMMITTED allows reading "dirty" data, including data locked by other transactions, but this level of isolation is not recommended due to dirty reads).

   -- Example of changing isolation level (depends on the DBMS)
   SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
   -- Then your SELECT query
   

2. Error handling: In a Python application, handle exceptions that occur when attempting to read locked fields. This allows retrying the query later or taking alternative actions.

   import psycopg2 # Example for PostgreSQL
   
   try:
       conn = psycopg2.connect(...)
       cur = conn.cursor()
       cur.execute("SELECT col1, blocked_col, col3 FROM your_table;")
       results = cur.fetchall()
       conn.commit()
   except psycopg2.OperationalError as e:
       # Handle lock error (depends on specific DBMS error)
       print(f"Error: {e}")
       # Possibly log, wait, and retry
   finally:
       if conn:
           cur.close()
           conn.close()
   

3. Using FOR UPDATE/FOR SHARE with NOWAIT/SKIP LOCKED: Some DBMSs (e.g., PostgreSQL, Oracle, MySQL with InnoDB) provide extensions to SELECT for managing locks when reading with the intention of subsequent modification or shared use.

   Keyword	Action
   NOWAIT	Immediately returns an error if a lock is detected.
   SKIP LOCKED	Skips locked rows if a lock is detected.

   -- Example of using SKIP LOCKED (PostgreSQL)
   SELECT col1, col2 FROM your_table WHERE condition FOR UPDATE SKIP LOCKED;
   
   -- Example of using NOWAIT (PostgreSQL)
   SELECT col1, col2 FROM your_table WHERE condition FOR UPDATE NOWAIT;
   

   SKIP LOCKED is the closest concept to "skipping locked fields," but it skips entire rows, not individual fields within a row.

4. Schema or logic modification: If locking fields is a frequent problem, consider revising the database schema or application logic to minimize lock durations or split data.

There is no direct way to make SELECT skip only one locked field but read other fields of the same row in standard SQL. Lock handling occurs at the row or table level, and the strategy depends on the specific DBMS and application requirements.