Re: "create table as" statement versus an "insert into" statement (performance

Truncating a table and releasing storage (the default) resets the high water mark. When you do a direct load or create table as select from, Oracle does not bring the rows through the buffer cache. It is the fastest way to move rows around. In a direct load or CTAS, Oracle just builds blocks and places them in the table, not bothering to check constraints until done. Further, Oracle does not go checking for each row, for a place to put it. Instead, it just appends the rows.

Direct loads of rows can be done unrecoverable/nologging. If unrecoverable/nologging, there will be a lot less redo generated. This also saves time. ----- Original Message -----
From: <Heidi_Schmidt_at_gillette.com>
To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com> Sent: Monday, October 23, 2000 10:01 PM
Subject: "create table as" statement versus an "insert into" statement (performance

>
> I could use some help.
>
> Does someone know why a create table as statement would perform better than
> a truncate table, and then insert into statement?
> Gut instinct tells me the create table is cleaner and the insert into has
> to find free chunks, but that can't be all of it.
>
> The background is that we have some developers who are executing large data
> loads and have to recreate objects every evening as part of a hierarchical
> OLAP tool process. They are looking to shave time wherever they can to get
> it all into their batch window.
>
> I am not sure where to begin to look for information that would show why
> one way would perform better than the other.
> I think it would be under parsing algorithms, which would be cool to learn
> if there was a list of them or a chart.
>
> Thanks in advance,
>
> Heidi
>
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Received on Tue Oct 24 2000 - 08:25:25 CDT