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IDS Forum

Re: Update Statistics & dbspace not clearing

Posted By: DL Redden
Date: Wednesday, 10 August 2005, at 12:55 a.m.

In Response To: Re: Update Statistics & dbspace not clearing (ART KAGEL, BLOOMBERG/ 731 LEXIN)

I was under the impression that BTREE cleaners are
"started up" when an UPDATE STATISTICS was ran. Is
this absolutely incorrect?

I seem to remember a server at my old job that had a
major performance issue one day. We were able to trace
the naughty thread down to the BTREE cleaner. The
Informix ESE's that were on site gave us an
undocumented onmode command to turn it off. I'm pretty
sure that they said an UPDATE STATISTICS would start
it up. It made since at the time beacuse normally
UPDATE STATISTICS did not run on that server and we
found a programmer who had executed one.

--- "ART KAGEL, ...." <KAGEL@bloomberg.net> wrote:

>
> Let me get this straight... Your programmer says
> that the table(s) and indexes
> are growing without limit because the space from
> deleted rows is not reused for
> new data within the tables? And he says that if he
> changes the app to run
> update statistics periodically it will release the
> space? Is that the question?
>
> Answer: No. You cannot find this documented because
> that's not the way that IDS
> works! Within a table and its indexes all deleted
> row space is reused
> immediately for newly inserted rows if needed with
> the sole exception of
> blobspace blob pages (TEXT or DATA type columns
> stored in blobspace). The
> pre-image copies of blobspace blobpages are not
> logged so they cannot be
> released until the next ontape/onbar archive has
> been taken. Update statistics
> has no effect on BLOB pages.
>
> OK, physically indexes are cleaned asynchronously by
> the BTREE CLEANERS or
> SCANNERS depending on the version. These threads
> compress partial nodes and
> free index pages thus emptied. Space freed by key
> deletions that produce
> partially filled index pages is not available
> immediately for new index pages
> but are freed over time. Completely empty index
> nodes are freed for reuse
> immediately and deleted key slots can always be
> reused before compression for
> new individual keys that fall into such a free slot.
> You can tune the BTREE
> scanners and can launch a scan using onmode. Update
> statistics has nothing to
> do with it.
>
> It is true that deleted row/key space is not
> released back to the server's free
> extent list for reuse by other tables. But update
> statistics has nothing to do
> with that either. To release unneeded space within
> a table or its indexes back
> to the free extent pool you have to reorganize the
> table and/or rebuild the
> index. Tables are reorged by one of the following
> methods:
>
> 1 - Unload the data, drop the table, recreate the
> table, reload the data,
> rebuild indexes.
> 2 - ALTER INDEX <some index on the table> TO
> CLUSTER;
> (if the index is already clustered uncluster it
> first. Requires space for
> the intermediate copy of the table, locks and
> logical log pages unless you
> alter the table to RAW first.)
> 3 - ALTER FRAGMENT FOR TABLE <sometable> INIT IN
> <dbspace or fragmentation
> clause>;
> (this can be done whether the table is
> fragmented or not and the dbspace or
> fragmentation clause can be a different one than the
> one the table already lives
> in or the same one. Requires space for the
> intermediate copy of the table,
> locks and logical log pages unless you alter the
> table to RAW first.)
>
> Art S. Kagel
>
> ----- Original Message -----
> From: John Whitte....
> At: 8/ 9 12:40
>
> 9.30.UC5 on Solaris 8
>
> A C++ application has it's own idx and dbs spaces
> and is unable
> to reclaim space from deleted rows. This causes the
> spaces to
> fill over time. The programmer says that this
> occurs because
> 'update statistics' must be run periodically on the
> database.
> He is building 'update statistics' into his
> application so space
> will be reclaimed in the dbspaces.
>
> I've never seen a database behave this way. I
> looked at my docs
> and 'Informix IDS Unlocking the Mysteries Behind
> Update
> Statistics' John F. Miller III, and found no mention
> of 'update
> statistics' in this context. I also find no info
> about reclaiming
> space being a problem in any context.
>
> Has anyone seen this problem? What kinds of things
> should I look
> for in the code?
>
> Thanks,
>
> John
>
>
>
>
>

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