Re: When are projections done?

From: Jonathan Lewis <>
Date: Tue, 2 Mar 2010 20:25:42 -0000
Message-ID: <>

"jbdhl" <> wrote in message
> Consider a table and a query referring to only a subset of the columns
> in that table. How early in the query evaluation is the projection
> carried out?
> Are the columns to be selected filtered out as early as in the very
> access method that reads the table rows from the buffer, or are the
> projection handled later, after the whole row has been fetched by the
> access method?
> Does it depend on the complexity of the query, how far down the three
> that the projection is handled out?
> Thanks!

At the earliest possible moment.

If you use dbms_xplan, one of the less well-known format options is "advanced" which will give a list of the columns projected at each line of the plan. (Funnily enough, the "advanced" option gives more data than the "all" option.)


Jonathan Lewis
Received on Tue Mar 02 2010 - 14:25:42 CST

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