To ensure database consistency and to support appropriate isolation levels between transactions, SQL Anywhere uses the following types of locks:
Schema locks These locks control the ability to make schema changes. 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.
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, executing a LOCK TABLE ... IN EXCLUSIVE MODE or REFRESH MATERIALIZED VIEW ... WITH EXCLUSIVE MODE statement acquires an exclusive table lock.
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.
Schema locks provide a mechanism to prevent schema changes from inadvertently affecting executing transactions. Row locks, table locks, and position locks each have a separate purpose, but they do interact. Each lock type prevents a particular set of inconsistencies. Depending on the isolation level you select, the database server uses some or all these lock types to maintain the degree of consistency you require.
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