A shared schema lock is acquired when a transaction refers directly or indirectly to a table in the database. Shared schema locks do not conflict with each other; any number of transactions can acquire shared schema locks on the same table at the same time. The shared schema lock is held until the transaction completes via a COMMIT or ROLLBACK.

Any connection holding a shared schema lock is allowed to change table data, providing the change does not conflict with other connections. The table schema is locked in shared (read) mode.

An exclusive schema lock is acquired when the schema of a table is modified, usually through the use of a DDL statement. The ALTER TABLE statement is one example of a DDL statement that acquires an exclusive schema lock on a table before modifying it. Only one connection can acquire an exclusive schema lock on a table at any time—all other attempts to lock the table's schema (shared or exclusive) are either blocked or fail with an error. A connection executing at isolation level 0, which is the least restrictive isolation level, is blocked from reading rows from a table whose schema has been locked in exclusive mode.

Only the connection holding the exclusive table schema lock can change the table data. The table schema is locked for the exclusive use of a single connection.