How Oracle Does It. No Read Locks

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Jennifer Cross
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1 How Oracle Does It Oracle Locking Policy No Read Locks Normal operation: no read locks Readers do not inhibit writers Writers do not inhibit readers Only contention is Write-Write Method: multiversion read consistency 1

2 Read always possible Reader does not see changes made by uncommitted transactions Reader does not see changes made by transactions (committed or not) after query starts. Reader sees only writes committed at time the query started. Basic Concepts System Log System Change Number (SCN) SCN used to determine transaction start order Space available for copying blocks in contention. 2

3 Writing process (220) copies original block to Then changes rows in original block Then Commits Block Timestamp New data Multiversion Read While transaction 220 (the writer) is active Transaction 225, a reader, begins. It can read even though 220 is changing the data item Because a copy of the original block is kept in the 3

4 220 Not Yet Committed Multiversion Read Reader New data will read Readers and Writers If Reader wants data that writer is writing, it uses rollback segment. If Writer wants data that reader is reading, reader directed to rollback segment. Reader gets consistent view: Committed data as of start of read. 4

5 220 Will Write to 102 Multiversion Read New data Reader Read now associated with will read Isolation Level of Statement Serializable: whole transaction has consistent view of data as of time transaction started. Read Committed: whole SQL statement (read, update, insert, delete) affecting several rows has consistent view of data. Read Only: whole transaction has view from single point in time. 5

6 Read Committed Update Default Oracle isolation level Wait for blocking writer. If statement tries to modify rows already modified by a committed later-starting statement: Restart the statement. Note: no program logic involved. Database handles automatically. Read Committed Update 230 will update Writer Updates 2 Rows 235 updates 103: writelock on Waits for 235 to complete. On abort, 230 continues 230 Automatically rolled back & restarted Row Timestamp updates just 103 Writer 235 6

7 Why is Restart Automatic? 230 a single SQL statement. Process gave 230 to DB to execute. No logical dependence between this statement and another. So statement can be executed at any time. Serializable Update If statement tries to modify rows already modified by committed laterstarting statement: Send error to program (exception) Program can Abort and roll back Quit or attempt to restart Roll back to savepoint Repeat last part of transaction 7

8 Serializable Update 230 will update Writer 230 Could not serialize 230 Updates 2 Rows Meanwhile 235 updates 103 Row Program Logic decides what to do next Timestamp updates just 103 Writer Waits for 235 to commit 230 Rolled back and gets error message Data Handled in Blocks Oracle Block may contain several OS blocks. Locks (row level) kept in block with row. Transaction timestamps: System Change Number (SCN) kept in block Recent history of block (SCNs) kept in block 8

9 Errors Write-write violations for serializable transactions. segment does not contain enough data Error: Snapshot too old Block does not contain enough recent history Too many transactions access same block Error: Cannot serialize Deadlock Rare since writers do not block readers and vice versa. Only occurs when 2 transactions access the same two rows for writing. Oracle uses deadlock detection. One transaction aborted and sent an error message. 9

10 FYI: SQL statements SET TRANSACTION ISOLATION LEVEL <level> ALTER SESSION SET ISOLATION_LEVEL <level> LOCK TABLE <table> IN <mode> SELECT FOR UPDATE 10

DATABASE TRANSACTIONS. CS121: Relational Databases Fall 2017 Lecture 25

DATABASE TRANSACTIONS. CS121: Relational Databases Fall 2017 Lecture 25 DATABASE TRANSACTIONS CS121: Relational Databases Fall 2017 Lecture 25 Database Transactions 2 Many situations where a sequence of database operations must be treated as a single unit A combination of