We see one such collision about every four years in the application
sites who share their data with us. The hash_value is definitely not a
unique identifier.
Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
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-Original Message-
Faroult
Sent: Wednesday, September 04, 2002 4:00 PM
To: Multiple recipients of list ORACLE-L
Pablo Rodriguez wrote:
I see 2 different addreses for the same hash_value in
v$sqlarea.
Why do I see this?
Here's an example:
hash_value address
-- ---
3749804 4064082C
3749804 4192941C
I wonder if someone can shed some light on this
Txs
My understanding is that to quickly find an already parsed statement in
memory, Oracle uses a hash function, which, like all hash functions is
not surjective (which means that you can have x y and f(x)= f(y), in
which case you say you have a collision). Of course once the hash value
has been computed values are checked, and all strings hashing to the
same value are linked - and stored at different addresses. You can also
have the case in which statements are strictly identical, but, for
session-related reasons, cannot be considered the same cursor (because
of synonyms pointing to different objects, of similarly named tables in
different schemas, or because optimizer settings have been altered at
the session level, typically). In that case it's normal to have the same
hash result for the same string - but you nevertheless have a collision
of a sort.
--
HTH,
Stephane Faroult
Oriole Software
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