RE: Undo Usage and Read consistency - ORA-1555
- From: Nancy Iles <nancy_iles@xxxxxxxxxxx>
- To: <cicciuxdba@xxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 14 Jul 2009 10:35:44 -0500
We have sporadic ora-1555 on an exceptionally simple statement that occurs
frequently in the application. How can you analyze why this tiny, simple
statement is causing an ORA-1555? The statement is:
SELECT RESV_NAME_ID , RESORT FROM RESERVATION_NAME WHERE CONFIRMATION_NO = :1
I believe that this causes a user session to hang. Our third party vendor says
that it is because a user terminates their session improperly and that it is
not an issue.
Any suggestions on how to analyze the cause and the system impact?
TIA,
Nancy Iles
Omni Hotels
________________________________
> Date: Tue, 14 Jul 2009 10:24:00 -0300
> Subject: Undo Usage and Read consistency - ORA-1555
> From: cicciuxdba@xxxxxxxxx
> To: oracle-l@xxxxxxxxxxxxx
>
> Dear DBAs,
>
> We got into a discussion about how read consistency is implemented in Oracle
> and was wondering what you know of this.
>
> The two sides are the following:
> 1) Undo is used for any and all selects, so if you do a full scan on a large
> table it is bound to give ORA-1555 even if there is no transaction modifying
> the table
>
> 2) Undo is only used when a transaction modifies data (DML) and ONLY then
> ORA-1555 is possible, since it happens when the consistent version of the
> block stored in the UNDO by the transaction ages out.
>
> Documentation is unclear to this respect:
>
> From the concepts guide:
>
> To manage the multiversion consistency model, Oracle must create a
> read-consistent set of data when a table is queried (read) and
> simultaneously updated (written). When an update occurs, the original
> data values changed by the update are recorded in the database undo
> records. As long as this update remains part of an uncommitted
> transaction, any user that later queries the modified data views the
> original data values. Oracle uses current information in the system
> global area and information in the undo records to construct a
> read-consistent view of a table's data for a query.
>
> Does this mean that every time I perform a select I get a copy of the data
> into de undo?
>
>
> Alan Bort
> Oracle Certified Professional
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