Select on view is long - Select on view definition short.

Question

Greetings Everyone -
There is a view in Oracle called V$LOCK.
If I do a count or a select on this view I get a result in *45 seconds*.
There are only 77 records.
However - If I do a select or a count on the view definition (found in V$FIXED_VIEW_DEFINITION)
I get a result in no time. Immediately.
I am signed in accordingly 'sqlplus / as sysdba'
What's going on ??? My feable brain cannot comprehend!!!
Thanks for any help,
BB

Answer 1

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There is a view in Oracle called V$LOCK.
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On 11.2 that is not correct. A query of 'select * from dba_synonyms where synonym_name like '%LOCK%' shows
V$_LOCK => V_$_LOCK
V$LOCK => V_LOCK
GV$_LOCK => GV_$_LOCK
GV$LOCK => GV_$LOCK
And a query of 'SELECT * FROM DBA_OBJECTS WHERE OBJECT_NAME LIKE '%LOCK%' shows
GV_$LOCK => VIEW
GV_$_LOCK => VIEW
V_$LOCK => VIEW
V_$_LOCK => VIEW
So V$LOCK is a synonym. And you may be querying the correct underlying view for each of your queries but sometimes the queries may not be on the view that you think they are.
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If I do a count or a select on this view I get a result in 45 seconds.
There are only 77 records.
However - If I do a select or a count on the view definition (found in V$FIXED_VIEW_DEFINITION)
I get a result in no time. Immediately.
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Let's see the first time you query data (when maybe there is nothing in the cache) it is slower than the second time you query the same data (when maby there is something in the cache)?
That isn't unusual.
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Well - that is interesting. Still it took you 11 seconds for 15 records !!!
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Records? There aren't any 'records'. The V$s are 'dynamic' performance views not tables. And read consistency is not guaranteed for them. The data is continuously updated so if there are no updates the view data won't change and depending on the updated data the view data may barely change.
It is unlikely that the data changed much, if at all, between your two queries.
See 'About Dynamic Performance Views' in the Database doc
http://docs.oracle.com/cd/B28359_01/server.111/b28320/dynviews_1001.htm#i1398692
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Oracle contains a set of underlying views that are maintained by the database server and accessible to the database administrator user SYS. These views are called dynamic performance views because they are continuously updated while a database is open and in use, and their contents relate primarily to performance.
Although these views appear to be regular database tables, they are not. These views provide data on internal disk structures and memory structures.
You can select from these views, but you can never update or alter them.
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And see the NOTE just below (as well as the section that discuss the V$ and GV$ views
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Because the information in the V$ views is dynamic, read consistency is not guaranteed for SELECT operations on these views.