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Posted by gilles27 on 10/18/05 12:29
Erland,
You're right to say that Ledger is the "main" key. I should have
explained but it looks like you figured it out yourself. I also
incorrectly stated the current column order in the PK. It is Ledger,
OrderLineNumber, OrderNumber and NOT Ledger, OrderNumber,
OrderLineNumber.
If I use your hierarchy theory the best order is Ledger, OrderNumber,
OrderLineNumber. This also fits in nicely with Joe's statement that "an
index (a, b, c) in effect gives you indexing for (a, b) and (a)",
because a common SELECT or JOIN on this table would involve Ledger and
OrderNumber as this points to the PK in the Orders table.
I am running SQL Profiler against a customer's database today so I hope
to have more information about the queries that our users run. This
will enable me to make a more informed decision about the sequence of
the columns in each PK in the database. It seems to me that it is more
important to understand usage of the database rather than ordering them
by number of distinct values.
Ross
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