When are projections done?

From: jbdhl <jbirksdahl_at_gmail.com>
Date: Tue, 2 Mar 2010 03:10:09 -0800 (PST)
Message-ID: <a6342b7f-315f-4e33-b6b7-c5d0a35724c3_at_t23g2000yqt.googlegroups.com>

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! Received on Tue Mar 02 2010 - 05:10:09 CST