Schema locks   These locks control the ability to make schema changes. For example, a transaction can lock the schema of a table, preventing other transactions from modifying the table's structure.

Row locks   These locks are used to ensure consistency between concurrent transactions at a row level. For example, a transaction can lock a particular row to prevent another transaction from changing it, and a transaction must place a write lock on a row if it intends to modify the row. To maximize concurrency, the key and non-key portions of the row can be locked independently. Updating non-key columns of a row does not interfere with the insertion and deletion of foreign rows referencing that row.

Table locks   These locks are used to ensure consistency between concurrent transactions at a table level. For example, a transaction that is changing the structure of a table by inserting a new column can lock a table so that other transactions are not affected by the schema change. In such a case, it is essential to limit the access of other transactions to prevent errors.

Position locks   These locks are used to ensure consistency within a sequential or indexed scan of a table. Transactions typically scan rows using the ordering imposed by an index, or scan rows sequentially. In either case, a lock can be placed on the scan position. For example, placing a lock in an index can prevent another transaction from inserting a row with a specific value or range of values.