The execution of a DDL or DCL request causes the data dictionary to be updated and appropriate locks to be placed on system tables while that request is processing.
Optimizing the Locking Granularity for Data Dictionary Access
To improve concurrency, DDL and DCL processing use the finest locking granularity that is practical and delay placing their locks for as long as possible. Depending on the dictionary table, the system sometimes downgrades rowhash READ lock requests made on the dictionary to ACCESS locks if the query would otherwise be blocked by WRITE locks placed on those tables by ongoing DDL operations.
If these rowhash READs are not blocked, then they use the standard READ locks.
The following dictionary views and tables are affected by this locking downgrade on a blocked READ lock request:
The only SQL statements eligible for a dictionary access READ lock-to-ACCESS lock downgrade upon being otherwise blocked are the following:
- HELP COLUMN
- HELP CONSTRAINT
- HELP INDEX
- HELP STATISTICS
- SHOW FUNCTION/HASH INDEX/JOIN INDEX/MACRO/METHOD/PROCEDURE/TABLE/TRIGGER/TYPE/VIEW
These are system-initiated lock downgrades: you cannot specify them using the LOCKING request modifier (see “LOCKING Request Modifier” in SQL Data Manipulation Language).