When will query data be blocked for better performance?

The query data will be blocked if none of these conditions are true.

Unless you force single-row protocol with the BLOCKING NO precompile/bind option, blocking will occur in both of the following cases:

The cursor is read-only (see note 3).
All of the following are true:
- There is no FOR UPDATE OF clause in the SELECT, and
- There are no UPDATE or DELETE WHERE CURRENT OF statements against the cursor in the program, and
- Either the program does not contain dynamic SQL statements or BLOCKING ALL was used.

Notes:

A cursor is updatable if it is not read-only (see note 3), and one of the following items is true:
- The select statement contained the FOR UPDATE OF clause, or
- There exists in the program an UPDATE or DELETE WHERE CURRENT OF against the cursor.
A cursor is potentially updatable if it is not read-only (see note 3), and if the program includes any dynamic statement, and the BLOCKING UNAMBIG precompile or bind option was used on SQLPREP or SQLBIND.
A cursor is read-only if one or more of the following conditions is true:
- The DECLARE CURSOR statement specified an ORDER BY clause but did not specify a FOR UPDATE OF clause.
- The DECLARE CURSOR statement specified a FOR FETCH ONLY clause.
- One or more of the following conditions are true for the cursor or a view or logical file referenced in the outer subselect to which the cursor refers:
  - The outer subselect contains a DISTINCT keyword, GROUP BY clause, HAVING clause, or a column function in the outer subselect.
  - The select contains a join function.
  - The select contains a UNION operator.
  - The select contains a subquery that refers to the same table as the table of the outer-most subselect.
  - The select contains a complex logical file that had to be copied to a temporary file.
  - All of the selected columns are expressions, scalar functions, or constants.
  - All of the columns of a referenced logical file are input only.