Issues with updating many rows oracle

Multiversion Concurrency Control To handle simultaneous access of the same data by many users, Oracle utilises various types of locks and a multiversion consistency model to maximise performance.

issues with updating many rows oracle-34issues with updating many rows oracle-53

Session Default You can set the isolation level of a transaction by using SET TRANSACTION… Oracle doesn't ever escalate row locks to block or table locks.

Oracle can maintain several different types of row lock: Exclusive lock mode - Prevents the resource from being shared. Only one exclusive lock can be placed on a resource (such as a row or a table).

When an UPDATE occurs, the original data values changed by the update are recorded in the database's undo records.

As long as this update remains part of an uncommitted transaction, any user that later queries the data will view the original uncommitted data.

Example Taxi table: Taxi_id, drivers_name jobs table: Job_id, Taxi_id, start_time, end_time Application inserts row into jobs for a Taxi_id Application looks in jobs table for overlaps with that row - "is the taxi already booked during that time?

" Two people - at the same time - will not see each others insert = double booking!

However, hold long-tern locks can cause two types of performance problems: SET LINESIZE 132 SET PAGESIZE 60 COLUMN OBJECT HEADING 'Database|Object' FORMAT a15 truncate COLUMN lock_type HEADING 'Lock|Type' FORMAT a4 truncate COLUMN mode_held HEADING 'Mode|Held' FORMAT a15 truncate COLUMN mode_requested HEADING 'Mode|Requested' FORMAT a15 truncate COLUMN sid HEADING 'Session|ID' COLUMN username HEADING 'Username' FORMAT a20 truncate COLUMN image HEADING 'Active Image' FORMAT a20 truncate SPOOL /tmp/locks SELECT c.sid, substr(object_name,1,20) OBJECT, c.username, substr(c.program,length(c.program)-20,length(c.program)) image, DECODE(b.type, 'MR', 'Media Recovery', 'RT', 'Redo Thread', 'UN', 'User Name', 'TX', 'Transaction', 'TM', 'DML', 'UL', 'PL/SQL User Lock', 'DX', 'Distributed Xaction', 'CF', 'Control File', 'IS', 'Instance State', 'FS', 'File Set', 'IR', 'Instance Recovery', 'ST', 'Disk Space Transaction', 'TS', 'Temp Segment', 'IV', 'Library Cache Invalidation', 'LS', 'Log Start or Switch', 'RW', 'Row Wait', 'SQ', 'Sequence Number', 'TE', 'Extend Table', 'TT', 'Temp Table', b.type) lock_type, DECODE(b.lmode, 0, 'None', /* Mon Lock equivalent */ 1, 'Null', /* NOT */ 2, 'Row-SELECT (SS)', /* LIKE */ 3, 'Row-X (SX)', /* R */ 4, 'Share', /* SELECT */ 5, 'SELECT/Row-X (SSX)', /* C */ 6, 'Exclusive', /* X */ to_char(b.lmode)) mode_held, DECODE(b.request, 0, 'None', /* Mon Lock equivalent */ 1, 'Null', /* NOT */ 2, 'Row-SELECT (SS)', /* LIKE */ 3, 'Row-X (SX)', /* R */ 4, 'Share', /* SELECT */ 5, 'SELECT/Row-X (SSX)', /* C */ 6, 'Exclusive', /* X */ to_char(b.request)) mode_requested FROM sys.dba_objects a, sys.v_$lock b, sys.v_$session c WHERE a.object_id = b.id1 AND = AND OWNER NOT IN ('SYS','SYSTEM'); Database Lock Mode Mode Session Active Object Type Held Requested ID Username Image ------------------------------------------------------------------------- 102 BANK OPS$MCACIAT @xdc (Pipe Two-Task DML Row-SELECT (SS) None 57 INVOICE OPS$DMAHTROB @xdc (Pipe Two-Task DML Row-X (SX) None 57 LINE_ITEM OPS$DMAHTROB @xdc (Pipe Two-Task DML Row-X (SX) None 70 LINE_ITEM OPS$JCONVICK @xdc (Pipe Two-Task DML Row-X (SX) None 29 LINE_ITEM OPS$NUMNUTS @xdc (Pipe Two-Task DML Row-X (SX) None 70 LINE_ITEM OPS$JKONV @xdc (Pipe Two-Task DML Row-X (SX) None 57 LINE_ITEM OPS$DMAHTROB @xdc (Pipe Two-Task DML Row-X If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts.

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Oracle guarantees that the set of data seen by a statement is consistent with respect to a single point in time and does not change during statement execution.

This means that applications which attempt to perform or enforce database consistency at the application level cannot assume the Oracle data they read will remain unchanged during the execution of a transaction (just because such changes are not visible to the transaction does not mean they don't happen).

Transaction-Level Read Consistency Oracle also offers the option of enforcing transaction-level read consistency.

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