Column Order
The order in which columns are listed in the index definition is important. It is possible to retrieve a set of row identifiers using only the first indexed column. However, it is not possible or efficient (on most databases) to retrieve the set of row identifiers using only the second or greater indexed column.
For example, imagine a phone book that is organized by city first, then by last name, and then by first name. If you are given the city, you can easily extract the list of all phone numbers for that city. However, in this phone book it would be very tedious to find all the phone numbers for a given last name. You would have to look within each city's section for the entries with that last name. Some databases can do this, others just won’t use the index.
Read more about this topic: Database Index
Famous quotes containing the words column and/or order:
“I stand on top
of our back steps and breathe the rich air—
a mother skunk with her column of kittens swills the garbage pail.
She jabs her wedge-head in a cup
of sour cream, drops her ostrich tail,
and will not scare.”
—Robert Lowell (1917–1977)
“The intellectual is a middle-class product; if he is not born into the class he must soon insert himself into it, in order to exist. He is the fine nervous flower of the bourgeoisie.”
—Louise Bogan (1897–1970)