Tom Lane wrote:
"Joshua D. Drake" <[EMAIL PROTECTED]> writes:
I guess where I got confused is:

And explicitly:

A multicolumn B-tree index can be used with query conditions that involve any subset of the index's columns, but the index is most efficient when there are constraints on the leading (leftmost) columns.

Considering the paragraph from the documentation above, should we change the documentation?

That statement seems perfectly accurate to me.

O.k. then perhaps I am being dense, but that statement says to me that the planner should be able to use the right element of a composite index but that it will not always do so.

Considering an index of a,b if I search for b I would expect that the planner could use the index. Assuming of course that the planner would use the same index if it was just b.

Further, I would expect a smaller chance of it using b if the index was a,c,b but that it "might" still use it.

Is that not the case? Should I expect that even in the simplest of cases that we will not use an index unless it is *the* leftmost element?


Joshua D. Drake

                        regards, tom lane

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