Date: Thu, 31 Oct 2002 14:39:12 -0500
Reply-To: Sigurd Hermansen <HERMANS1@WESTAT.COM>
Sender: "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From: Sigurd Hermansen <HERMANS1@WESTAT.COM>
Subject: Re: SQL Question - Doing equijoins and getting this log message
Content-Type: text/plain; charset="iso-8859-1"
Summary: Avoid solutions that require Cartesian products for matching more
than a few thousand records. Contact Paul Dorfman if you need a reasonable
solution to a large-scale matching problem.
A thread that touched indirectly on this topic wove its way through the list
not too long back. The message means, in basic terms, that the SQL compiler
cannot determine an index or sort order of the keys (LineID) that it can use
to limit the rows being compared to resolve the where condition. Sometimes
that means that none of the typical optimization strategies will work, and
sometimes it means that the SAS SQL compiler cannot implement whatever
strategies might work.
The inequality operator (<) in the WHERE clause is complicating the SQL plan
for the query. I believe that this query represents an extreme case of a
theta-join. I consider it an extreme case because the query should select
about half of the Cartesian product of scenarios reflexed on itself! That
means that if scenarios has 5,000 rows, you would expect the query to yield
12.5 million rows. The numbers increase geometrically with the number of
rows. SAS SQL may as well in this case form the Cartesian product and throw
out the ones that do not meet the WHERE condition.
Inexact matching of natural key values may require comparisons of every
possible pairing of two sets of records. If the decision to select a match
depends on the yield of a binary function with one argument being taken from
each record, how else could a program determine which pairs of records to
select? Binary comparison methods for deduplicating large sets of records
tend to bog down quickly as the number of records increases.
The solution? No perfect solution exists for this class of problem, but
reasonable approximations involving prescreening on indexed unary function
values cost little in terms of sensitivity and allow control of specificity
of matching. If anyone has to find a way to conduct fuzzy matching or
deduplication of a large number of records, send an e-mail to Paul Dorfman.
He has developed a very fast and accurate implementation of unary function
value indexing, and he understands the in's and out's of fuzzy linkage.
From: Gerstle, John [mailto:yzg9@CDC.GOV]
Sent: Thursday, October 31, 2002 11:54 AM
Subject: SQL Question - Doing equijoins and getting this log message
I'm looking for some more information and an understanding of this message
in the log after running an sql program, matching each row of a table with
itself using a WHERE statement that prevents a row from matching with itself
or others before it in the order of the original rows (i.e. line1 > line2).
It's a modified program from the fuzzy matching programs put up on Charles
create table matches as
select t1.lineID as lineID1,t2.lineID as lineID2,
t1.idno as idno1, t2.idno as idno2,
from scenarios as t1, scenarios as t2
where t1.lineID < t2.LineID ;
I've searched the SAS-L archives and SAS Tech Support, and found some
interesting tidbits, but not any info on this specific log message.
Here is the message:
NOTE: The execution of this query involves performing one or more Cartesian
product joins that can not be optimized.
Is this a worry? Can this be corrected, if so?
Thanks for any input and/or opinions.
CDC Information Technical Services (CITS)
Contractor Support to NCHSTP
Division of HIV/AIDS Prevention
HIV/AIDS Incidence and Case Study Branch (HICSB)
Phone: 404-639-3980 Fax: 404-639-2980
Cell: 770-639-5060 Email: firstname.lastname@example.org