 Re: referred more than once in access plan means?
"Mark A" <ma@switchboard.net> wrote in message
news:HevVb.489$Jj6.41784@news.uswest.net...[color=blue]
> "Fan Ruo Xin" <fanruox@sbcglobal.net> wrote in message
> news:7RuVb.2570$PY.839@newssvr26.news.prodigy.com. ..[color=green]
> > Can someone help me to confirm -
> > From the access plan tree of a query, if one table is referred (TABLE[/color]
> SCAN)[color=green]
> > three times. Does that mean this table was scanned (either from disk or
> > bufferpool) three times during runtime?
> > TIA
> > FRX
> >[/color]
> That is certainly possible, especially if there is a subselect. It might
> also indicate intra-partition or inter-partition parallelism. But it is[/color]
hard[color=blue]
> to know exactly what you are referring to without seeing the explain[/color]
output.
============
That is a good point. Parallel read might refer a table (especially a small
table) more times than a non-parallel read.
If I used neither intra_parallelism, nor inter_parallelism. I want to know
when you check the access plan, and you find out that a table is referred
three times. Does that mean this table need to be read three times?
[color=blue]
>
> All data is accessed by applications from the bufferpool. If the page[/color]
which[color=blue]
> contains the needed row is not in the bufferpool, DB2 puts it in the
> bufferpool for access by the application. The explain does not take into
> consideration any physical I/O to put needed pages in the bufferpool if[/color]
they[color=blue]
> are not already there. However, the explain will indicate prefetch[/color]
activity,[color=blue]
> which is attempt by DB2 to place pages in the bufferpool before they are
> asked for by the application.[/color]
=========
In fact, my question is not this part. Through take a view of access plan.
You can't determine if the read is physical or logical ...[color=blue]
>
>[/color]
| 
