Re: using temp tables for staging databases?

Hi Arup,

Having a low PCTUSED will achieve nothing with regard to row density per block if there are no deletes or really really significant updates that reduce row lengths. Therefore, using it to reduce BBW is a doubtful method.

That said, using say PCTFREE (for which a high value might be effective) to artificially reduce row density per block and hence possibly reduce BBW is also dangerous. Wasted space below the HWM and the extra LIOs associated with FTS can cause more issues than the BBWs you're trying to avoid. Especially if FTS are common and the BBWs are as a result of poor freelist management during inserts ...

In my opinion, the reason for the default settings for PCTFREE and PCTUSED is that a row must be greater than 50% of the block size for the insertion to fail whist the block is currently *under* the PCTUSED value (for which a new block is required). This would be the worst case scenario.

IMHO, a PCTUSED of 40 is dangerous when tables have sparse/random deletions as this could again result in wasted space below the HWM. FTS would just hate you for it. Ideally PCTFREE should be sized to accommodate average row growths, PCTUSED should be sized to efficiently reclaim deleted space without excessive freelist overheads.

It's easier said than done ;)

Cheers

Richard

• Original Message ----- To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Tuesday, October 21, 2003 10:29 PM

> Mark,
>
> While waiting for Tim, I can offer another situation - in datawarehouses,
> where the subsequent updates are not likely to occur. Also, space is a
> premium and packing the blocks as densly as populated might be necessary.
>
> I will also add to Tim's response of justifying a smaller PCTUSED. In
> addition to the freelist problem he mentioned, there is also a greater
> chance of buffer busy waits occuring when a block contains too many rows.
In
> an OLTP database that is certainly likely to happen - another case for the
> default 40 setting for the parameter. In DW, however, the chances of BBW
are
> low, hence a higher setting may be possible.
>
> HTH.
>
> Arup Nanda
>
> ----- Original Message -----
> To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com>
> Sent: Tuesday, October 21, 2003 5:19 AM
>
>
> > Tim,
> >
> > Can you sum up a few situations when the need *has* arisen to change
these
> > values?
> >
> > Cheers
> >
> > Mark
> >
> >
> >
> > -----Original Message-----
> > Tim Gorman
> > Sent: 21 October 2003 06:09
> > To: Multiple recipients of list ORACLE-L
> >
> >
> > Unless you typo'd, there are some serious problems here...
> >
> > Setting PCTFREE to 99 is not likely to "pack in the blocks". Rather the
> > opposite; you are instead leaving blocks 99% empty. Quite a bit of
> wasted
> > I/O in performing a FULL table scan here... :-)
> >
> > Anyway, it is not a good idea to have PCTFREE and PCTUSED sum to a value
> > greater than 70 or 80 or so, just as a rule of thumb. Having them sum
to
> a
> > value near 100 ensures that each insert, delete, or even update will
> > potentially cause the block to be removed or reinserted to one of the
> > segment's free list. Think about it: the width of a single row
crossing
> > the boundary from "off the free list" to "on the free list". Better to
> > leave a bit of a "no man's land" between the two values. The default
> > settings of PCTFREE=10 and PCTUSED=40 are one of the few default
settings
> > that need little manipulation for most situations.
> >
> >
> >
> > on 10/20/03 7:34 AM, rgaffuri_at_cox.net at rgaffuri_at_cox.net wrote:
> >
> > > we drop and recreate the temp tables every night. We also use PCTFREE
> > PCTUSED
> > > at 99 and 1 to pack in the blocks and we use very small extent sizes.
> then
> > we
> > > analyze with an estimate size of 20 percent which is quite fast.
> > >
> > > All of them are used for full table scans and do not have indexes. Ive
> > found
> > > that a 'create table as' is MUCH faster than inserting into global
> > temporary
> > > tables when you do not have to worry about latch contention(ie 1-3
users
> > > logged in at a time).
> > >
> > > anyone else notice this? Seems to go against conventional wisdom which
> > says
> > > never use them. So I want to make sure Im not missing something.
> > >>
> > >> From: Tim Gorman <tim_at_sagelogix.com>
> > >> Date: 2003/10/20 Mon AM 10:19:33 EDT
> > >> To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
> > >> Subject: Re: using temp tables for staging databases?
> > >>
> > >> All the time. Oracle Apps's "open interfaces" are built this way,
for
> > >> example.
> > >>
> > >> However, "the guys here" covered their bases by specifying "smaller
> > >> temporary tables", as if they could prevent them from becoming large.
> I
> > >> suppose they might feel that they indemnify themselves if the tables
> > should
> > >> ever become "large"?
> > >>
> > >> As with OraApps "open interface" tables, it is when a large volume of
> > data
> > >> is pushed through that the trouble starts. The "high-water marks" on
> all
> > >> the tables are pushed to a high level, thereafter causing full table
> > scans
> > >> on the interface/temporary tables to run slowly. The only way to
bring
> > the
> > >> HWM back down is quiesce the interface/app and then truncate the
> tables.
> > >>
> > >>
> > >>
> > >> on 10/20/03 6:39 AM, rgaffuri_at_cox.net at rgaffuri_at_cox.net wrote:
> > >>
> > >>> This is for non-transactional data load instances. The guys here
sware
> > that
> > >>> by
> > >>> using smaller temporary tables(not global temp tables) they can
> increase
> > the
> > >>> speed of the data loads.
> > >>>
> > >>> Not worried about latch contention because its just for bulk loads.
I
> > know
> > >>> this bad in transactional instances. Has anyone used these in
> > >>> non-transactional data load instances?
> > >>
> > >> --
> > >> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> > >> --
> > >> Author: Tim Gorman
> > >> INET: tim_at_sagelogix.com
> > >>
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> > Author: Tim Gorman
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> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Arup Nanda
> INET: orarup_at_hotmail.com
>
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Author: Richard Foote
  INET: richard.foote_at_bigpond.com

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