There is no limit to the wait time unless the table is remote. Without the NOWAIT clause, your process will block until the table is available. In this case, control will be returned immediately to your program so that you can perform other work or simply wait for a period of time before trying again. If you simply state FOR UPDATE in the query and do not include one or more columns after the OF keyword, then the database will then lock all identified rows across all tables listed in the FROM clause.įurthermore, you do not have to actually UPDATE or DELETE any records just because you issued a SELECT.FOR UPDATE - that act simply states your intention to be able to do so.įinally, you can append the optional keyword NOWAIT to the FOR UPDATE clause to tell Oracle not to wait if the table has been locked by another user. Locks are still placed on all rows the OF list just gives you a way to document more clearly what you intend to change. The OF list of the FOR UPDATE clause does not restrict you to changing only those columns listed. The FOR UPDATE OF clause only mentions the max_procrastination_allowed column no columns in the winterize table are listed. In the following example the FOR UPDATE clause does not result in any locked rows in the winterize table:įOR UPDATE OF husband_config.max_procrastination_allowed In this case, rows in a table are locked only if the FOR UPDATE clause references a column in that table. You can use the FOR UPDATE clause in a SELECT against multiple tables. The first cursor uses the unqualified FOR UPDATE clause, while the second cursor qualifies the FOR UPDATE with a column name from the query. SELECT task, expected_hours, tools_required, do_it_yourself_flag SELECT name, manufacturer, preference_level, sell_at_yardsale_flag Here are two examples of the FOR UPDATE clause used in a cursor: No one else will be able to change any of these records until you perform a ROLLBACK or a COMMIT. When you issue a SELECT.FOR UPDATE statement, the RDBMS automatically obtains exclusive row-level locks on all the rows identified by the SELECT statement, holding the records "for your changes only" as you move through the rows retrieved by the cursor. Oracle offers the FOR UPDATE clause of the SELECT statement to perform this locking. There are times, however, when you will want to lock a set of records even before you change them in your program. Even then, others will be able to read those records as they appeared before the change (the "before image" of the data). In general, this is a wonderful feature because the number of records locked at any given time is (by default) kept to the absolute minimum: only those records which have been changed but not yet committed are locked. RETURNING).Ī transaction cannot be ended inside a block with exception handlers.When you issue a SELECT statement against the database to query some records, no locks are placed on the selected rows. Transaction commands are not allowed in cursor loops driven by commands that are not read-only (for example UPDATE. The cursor is still removed automatically after the loop, so this is mostly invisible to the user. That means that the cursor is fully evaluated at the first COMMIT or ROLLBACK rather than row by row. However, a cursor created as part of a loop like this is automatically converted to a holdable cursor by the first COMMIT or ROLLBACK. Normally, cursors are automatically closed at transaction commit. Consider this example:įOR r IN SELECT * FROM test2 ORDER BY x LOOP Special considerations apply to cursor loops. But if the call stack is CALL proc1() → SELECT func2() → CALL proc3(), then the last procedure cannot do transaction control, because of the SELECT in between. For example, if the call stack is CALL proc1() → CALL proc2() → CALL proc3(), then the second and third procedures can perform transaction control actions. Transaction control is only possible in CALL or DO invocations from the top level or nested CALL or DO invocations without any other intervening command. The commands COMMIT AND CHAIN and ROLLBACK AND CHAIN accomplish this. In cases where transactions are committed in a loop, it might be desirable to start new transactions automatically with the same characteristics as the previous one. (Note that BEGIN and END have different meanings in PL/pgSQL.)Ī new transaction starts out with default transaction characteristics such as transaction isolation level. A new transaction is started automatically after a transaction is ended using these commands, so there is no separate START TRANSACTION command. In procedures invoked by the CALL command as well as in anonymous code blocks ( DO command), it is possible to end transactions using the commands COMMIT and ROLLBACK.
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