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At 05:24 PM 3/31/2004 -0500, you wrote:
On Tue, 2004-03-30 at 18:08, Vern Hamberg wrote:
> AFAIK, physical files are not the same as flat files. They ARE the same as
> tables created through the SQL CREATE TABLE statement.
>
Not entirely true. For example, there are two code paths used for
selecting from a table scan.
My comment was really in the context of differentiating from flat files.
The difference I meant to highlight is that PFs have a record structure
(fields defined as part of the PF), flat files, a la S36, do not - need to
be defined in a program. If that's what was meant by flat file.
Your point is well-taken, however, as my statement was too general. SQL
tables are physical files with some attributes added/different - DSPFD
shows the differences - still a physical file, but has an SQL attribute
that a flat file (created using RCDLEN, not DDS source) will not, but it is
externally described, just as a PF defined with DDS.
If you are selecting on a numeric value, and that table was built with
DDS, then the optimizer is forced to use the 'Derived field' selection.
If the table was built with SQL, then the table scan can use the Direct
selection. The code path is shorter on the Direct selection.
So this is one of the advantages Rick probably wants to know about, eh? Cool.
> And keyed logical files and SQL indexes are essentially the same thing -
> B-trees.
The Access Path part is, but most keyed logical files show up as views,
even when no mapping takes place. And sometimes, they don't even show up
as views. But the optimizer still considers non-sparse access paths
during the optimization phase.
Show up in OpsNav? I suppose - Hmm, that's interesting, looking at the
"description" item when I right-click. Although there is an access path
tab, it does not show the actual key fields - but it does show the key
length - certainly this could be displayed better? And the definition tab
can't generate a statement - of course, as it explains. Worth looking at
once, IMO.
The priority in the optimizer for looking at available access paths - is it
still based on age? I think that's what it was. (and probably other factors)
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