SQL functions in TOOL code

Answer 1

In my original email I should have made the point clear that an indexed
column was required, that led to some confusion, apologies.
Under Oracle 7 even if the column is indexed the query engine still does a
full scan of the index to find the maximum or minimum value. As strange as
this seems it is possible to view it using the Oracle trace functions such
as tkprof. This method is quicker than not having an index but the cursor
method is far more efficient.
When using a cursor based approach Oracle will go straight to the first
record of the index (depending on MAX or MIN) and retrieve the data. By
exiting at that point the function has been performed and the I/O operations
are extremely low compared to a full index scan.
Of course there is a trade off depending on the amount of rows but for large
indexed tables the cursor approach will be far faster than the normal
functions. I'm not sure how other RDBMS's handle MAX/MIN but this has been
my experience with Oracle. This process may be faster still by using PL/SQL
but then you are incorporating specific database languages which is
obviously a problem if you port to a different RDBMS. Here is some code you
can try for Oracle PL/SQL functions:
declare
cursor myCur1 is
select number_field
from number_table
order by number_field desc;
begin
open myCur1;
fetch myCur1 into :max_val;
close myCur1;
end;
I hope this clarifies things a bit more. If in doubt of the execution plan
of a performance critical query use the database trace functions as they
show up all sorts of surprises. MAX and MIN are easy to understand when
viewing code but perform poorly under Oracle, whether v8 behaves differently
I have yet to discover.
Cheers,
Dylan.
-----Original Message-----
From: [email protected] [mailto:[email protected]]
Sent: Thursday, 7 January 1999 3:37
To: [email protected]
Subject: RE: SQL functions in TOOL code
I guess my point is that MAX can always be implemented more
efficiently than the SORT/ORDER-BY approach (but may not be the
case, depending on the RDBMS). If an ORDER-BY
can use an index (which means that the indexing mechanism involves
a sorted collection rather than an unordered hashtable) so can
MAX - in which case finding a MAX value could be implemented
in either O(1) or O(logn) time, depending on the implementation.
The last sentence being the major point of this whole discussion,
which is that your mileage may vary depending on the RDBMS - so
try using both approaches if performance is a problem.
In terms of maintenance, MAX is the much more intuitive approach
(In My Opinion, of course), since a programmer can tell right away
what the code is attempting to do.
Chad Stansbury
BORN Information Services, Inc.
-----Original Message-----
From: [email protected]
To: [email protected]; [email protected]; [email protected]
Sent: 1/6/99 10:45 AM
Subject: RE: SQL functions in TOOL code
Well, yes, but in that specific case (looking for max() value) would not
be
true that, if you have an index (and only then) on that specific column
some
databases (like Oracle) will be smart enough to use index and find max
value
without full table scan and without using order by clause?
Dariusz Rakowicz
Consultant
BORN Information Services (http://www.born.com)
8101 E. Prentice Ave, Suite 310
Englewood, CO 80111
303-846-8273
[email protected]
-----Original Message-----
From: Sycamore [SMTP:[email protected]]
Sent: Wednesday, January 06, 1999 10:29 AM
To: [email protected]; [email protected]
Subject: Re: SQL functions in TOOL code
If (and only if) an index exists on the exact columns in the ORDER BY
clause, some databases are smart enough to traverse the index (inforward
or
reverse order) instead of doing a table scan followed by a sort.
If there is no appropriate index, you always end up with some kind ofsort
step.
Of course this is all highly schema- and database-dependent, so youmust
weigh those factors when deciding to exploit this behavior.
Kevin Klein
Sycamore Group, LLC
Milwaukee
-----Original Message-----
From: [email protected] <[email protected]>
To: [email protected] <[email protected]>
Date: Wednesday, January 06, 1999 9:40 AM
Subject: RE: SQL functions in TOOL code
This seems a bit counter-intuitive to me... primarily due to
the fact that both MAX and ORDER-BY functionality would require
a full table scan on the given column... no? However, I would
think that a MAX can be implemented more efficiently since it
just requires the max value in a given set (which can be performed
in O(n) time on an unordered set) versus an ORDER-BY (sort)
performance on an unordered set of at best O(nlogn) time.
Am I missing something? Please set me straight on this 'un.
Chad Stansbury
BORN Information Services, Inc.
-----Original Message-----
From: Jones, Dylan
To: 'Vuong, Van'
Cc: [email protected]
Sent: 1/5/99 4:42 PM
Subject: RE: SQL functions in TOOL code
Hi Van,
Operating a function such as MAX or MIN is possible as given in your
example
but it is worth pointing out the performance overhead with such a
method.
When you use MAX, Oracle will do a full table scan of the column so
if
you
have a great many rows it is very inefficient.
In this case use a cursor based approach and depending on your
requirments
(MAX/MIN) use a descending or ascending ORDER BY clause.
eg.
begin transaction
for ( aDate : SomeDateDomain ) in
sql select DATE_FIELD
from DATE_TABLE
order by
DATE_FIELD DESC
on session MySessionSO
do
found = TRUE;
aLatestDate.SetValue(aDate);
// Only bother about the first record
exit;
end for;
end transaction;
On very large tables the performance increases with the above method
will be
considerable so it is worth considering which method to use whensizing
your
database and writing your code.
Cheers,
Dylan.
-----Original Message-----
From: Vuong, Van [mailto:[email protected]]
Sent: Tuesday, 5 January 1999 6:50
To: [email protected]
Subject: SQL functions in TOOL code
Is it possible to execute a SQL function from TOOL code?
For example:
SQL SELECT Max(Version) INTO :MyVersion
FROM Template_Design
WHERE Template_Name = :TemplateName
ON SESSION MySession;
The function in this example is MAX().
I am connected to an Oracle database.
Thanks,
Van Vuong
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