The index may be used when either large number of deleted rows exist
or when the amount of memory to page is easily managed by index vs the
dataspace. That is because the index has the RRN [or RRN can be easily
calculated from], and that index can be more efficiently paged than the
entire dataspace with a full table scan. So when all of the information
is available in an index which is smaller than the data, it can be much
more efficient to use the index instead, especially when there is no
selection.
Note: To further clarify on the Join Logical File index... There is
not just one index for all three files. In the three file join, the
database creates the primary index as declared in the DDS, and then
creates additional indexes on the second and third file which assist in
effecting matches for join and selection. Given no selection, each
index created on each file is much like a CREATE INDEX which is used for
implementation of a dynamic join when the JLF is opened; just that there
is not a separate object, to which the index can be associated.
Regards, Chuck
Luis Rodriguez wrote:
I agree (and am very conscious of) with the fact that the only way to
guarantee a result ordered is with the ORDER BY clause.
That said, what I don't really understand is SQL using an index for
reporting the RRN(). One would imagine that kind of things would be
better doing a table scan.
Terrence Enger wrote:
Let me remind people--I confess, myself included--that if
you want results in a specific order, you must specify that order
explicitly. <<SNIP>>
Luis Rodriguez wrote:
Running some tests (V5R3) I found this strange (at least for me)
results with SQL RRN: (FILE is a (DDS) file with no index and
zero deleted records) Select RRN(file) from FILE;
<<SNIP unordered SELECT RRN(A) vs ordered SELECT RRN(A),A.* >>
Running the sentence under debug (STRDBG) shows that SQL decided
to use an index (LF). My question is: Why does RRN() needs an
index?. Moreover, the selected index is a join file(3 Files!!)
This is not giving any problem for me right now, I'm just curious
why this is so. Any ideas?
