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Kevin, I'm with you all the way until
"In any case, the sequential variety is probably the one you'd want to minimize."
Sticking to Oracle7 for the sake of simplicity. Why would I want to minimize sequential read waits when these indicate non-full index scans? I'd much rather minimize avoid full table scans, ie. scattered read waits.
Regards,
Ivan Bajon, ocp
"Kevin Brand" <kevin.brandx_at_tel.gte.com> wrote in message
news:93i0d7$i9f$1_at_news.gte.com...
>
> A few tests reveal the following:
>
> In Oracle7 v7.3.4:
>
> doing an index scan with V733_plans_enabled=false I get
> waits for sequential read
>
> doing a table scan with db_file_multiblock_read_count > 1 I get
> waits for scattered read
>
> doing a table scan with db_file_multiblock_read_count = 0 I get
> waits for sequential read
>
> In Oracle8 v8.1.6
>
> basically the same except that an index scan produces scattered read
waits.
> I believe this is true because index scans in O8 actually use read-ahead
by
> default ( FFS ).
>
> So, based on the above, sequential and scattered waits both are indicitave
> of table scans. It depends on whether or not the engine chooses ( based
on
> parameters ) to perform advanced reads ( read-ahead ). Depending on which
> version of the server you run, these wait events may also indicate index
> scans.
>
> In any case, the sequential variety is probably the one you'd want to
> minimize.
>
> -Kevin
>
> "Ivan Bajon" <bajon_at_nospam@worldonline.dk> wrote in message
> news:93h9gi$eof$1_at_news.inet.tele.dk...
> > Could someone please clarify whether full table scans cause waits for db
> > file sequential read or scattered reads. I've seen claims on both.
> >
> > Steve Adams says:
> > "Sequential reads occur mainly in connection with full table scans and
fast
> > full index scans." (Taken from Tips->Disk Configuration for Sequential
Scans
> > on Ixora)
> >
> > In "Description of Oracle7 Wait Events and Enqueues" (2.36 db file
scattered
> > read) Anjo Kolk says:
> > "This event is normally an indicator that a Full Table Scan is happening
on
> > the object identified before."
> >
> > This leaves me confused. Which is it? Both?
> >
> > Regards,
> > Ivan Bajon, ocp
> >
> >
>
>
Received on Wed Jan 10 2001 - 15:52:04 CST