When are projections done?
in [Oracle]
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From: jbdhl on 2 Mar 2010 06:10 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!
From: John Hurley on 2 Mar 2010 09:07 On Mar 2, 6:10 am, jbdhl <jbirksd...(a)gmail.com> wrote: snip > 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! Why does it matter? Any specific release you are investigating? Any specific problem query that you are trying to optimize?
From: Michel Cadot on 2 Mar 2010 11:04 "jbdhl" <jbirksdahl(a)gmail.com> a �crit dans le message de news: a6342b7f-315f-4e33-b6b7-c5d0a35724c3(a)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! Oracle does not fetch rows, it reads blocks matching the conditions you gave and then returns you selected columns when you fetch the rows. Regards Michel
From: Jonathan Lewis on 2 Mar 2010 15:25 "jbdhl" <jbirksdahl(a)gmail.com> wrote in message news:a6342b7f-315f-4e33-b6b7-c5d0a35724c3(a)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! 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.) http://jonathanlewis.wordpress.com/2008/03/06/dbms_xplan3/ -- Regards Jonathan Lewis http://jonathanlewis.wordpress.com
From: jbdhl on 2 Mar 2010 16:06
> At the earliest possible moment. Could that be directly inside the access method that fetches individual rows from the page in the buffer? Or is the projection always performed at some point *after* the initial fetch of the relevant row(s)? |