Oracle Apps DBA: Keeping E-Business Suite Alive · Module 3 · The database itself
Undo, and read consistency
Lesson 28 of 184 · 3 min
Why a long report can fail with an error about UNDO, and why the fix is USUALLY NOT A BIGGER UNDO TABLESPACE. Undo holds the previous version of every changed row, and it does two jobs. Rolling back a transaction, obviously — and READ CONSISTENCY, which is the one that produces the failure in the title. READ CONSISTENCY MEANS A QUERY SEES THE DATA AS IT WAS WHEN THE QUERY STARTED. A report running for an hour does not see rows that changed during that hour; it reconstructs the old version from undo. That is why Oracle readers do not block writers, and it is a genuinely valuable property rather than an implementation detail. SO A LONG QUERY DEPENDS ON UNDO IT DOES NOT OWN, GENERATED BY OTHER PEOPLE'S TRANSACTIONS. If the system is busy, that undo can be overwritten before the query finishes — and the query fails because…
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In this module: Module 3 · The database itself
- 1Instance and database: two different things
- 2Starting and stopping, properly
- 3Tablespaces and datafiles
- 4Redo: the reason recovery is possible
- 5Archive log mode, and the day it was off
- 6Undo, and read consistency
- 7Parameters, and the ones that matter here
- 8The data dictionary, as a diagnostic tool
- 9Reading an error you have never seenFree preview
- 10What breaks: four database-tier failures
- 11Lab briefing · Stop it, start it, break it