Re: When are projections done?
Date: Tue, 2 Mar 2010 17:04:26 +0100
"jbdhl" <jbirksdahl_at_gmail.com> a écrit dans le message de news: 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?
Oracle does not fetch rows, it reads blocks matching the conditions you gave and then returns you selected columns when you fetch the rows.
Michel Received on Tue Mar 02 2010 - 10:04:26 CST