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IDS Forum

RE: Poorly Optimized SQL

Posted By: Everett Mills
Date: Tuesday, 6 August 2013, at 3:17 p.m.

In Response To: Poorly Optimized SQL (MICHAEL HOFFMAN)

I can't test it, but I think this would accomplish the same task and be a bit easier for the optimizer to digest.

--EEM

UPDATE q SET reg_cncl_dt = s1.cancel_dt
FROM v_inq q
INNER JOIN

(

SELECT m.v_id::INTEGER FROM mthly_updts m, mthly_info s

WHERE m.v_id = s.v_id

AND m.typ = 'E'

AND m.cit = 'U'

AND s.status = 'C'

) d
ON d.v_id = q.v_id
AND q.vg_stat <> 'C'
INNER JOIN

(

SELECT MAX(cancel_dt) cancel_date, v_id

FROM mthly_info s1

GROUP BY v_id

) s1 ON s1.v_id = TO_CHAR(q.v_id)

> -----Original Message-----
> From: ids-bounces@iiug.org [mailto:ids-bounces@iiug.org] On Behalf Of
> MICHAEL HOFFMAN
> Sent: Tuesday, August 06, 2013 11:14 AM
> To: ids@iiug.org
> Subject: Poorly Optimized SQL [31052]
>
> Hi All,
> Can someone explain why Informix is going out to lunch on the 'set'
> subquery?
> The accessed table is the same one as in the main query. Almost
> immediately prior to running this statement, that table was "update
> statistics low drop distributions" and "update statistics high on
> v_id", so stats should be fine.
>
> Running Informix 11.50.FC6 on Solaris 10.
>
> I'm interested in *why* the Optimizer is basically ignoring the index
> on 'm1'
> and taking so long to run that subquery (as opposed to configuration
> changes to make it run faster; there are plenty of tuning issues to be
> addressed).
>
> v_inq is a permanent table with 8000 rows, residing in one dbspace.
> (v_id integer, primary key) mthly_info (s / s1) is a temp table created
> by selecting from an external table, 3M rows. (v_id varchar(50),
> indexed) mthly_updts (m) is a very small temp table holding a subset of
> ids and statuses, 9365 rows. (v_id varchar(50))
>
> One Temp dbspace configured (I know, but this is a dev box, short on
> space).
> PDQ Priority = 100 -- not in use, so not relevant.
>
> I would expect the Query to run in this fashion (please correct if I'm
> wrong):
> 1) Select v_ids between tables 'm' and 's'.
> 2) Update 'v_inq' records for those v_ids,
> 3) pulling associated cancel_dt from 's1' matching by v_id.
>
> So, why is the optimizer hitting s1 *49.8 Billion* times? (see Explain
> Plan) Even if it sequentially scanned s1 for every v_inq record, that
> is only 24B reads (8K * 3M).
>
> Thanks,
> Mike
>
> Here is the Explain Plan:
> QUERY: (OPTIMIZATION TIMESTAMP: 08-05-2013 16:17:20)
> ------
> update v_inq
> set reg_cncl_dt = (select unique cancel_dt from mthly_info s1
>
> where s1.v_id = to_char(v_inq.v_id))
> where v_id in (
> select unique m.v_id::integer from mthly_updts m, mthly_info s
>
> where m.v_id = s.v_id
>
> and m.typ = 'E' and m.cit = 'U'
>
> and s.status = 'C' )
> and vg_stat <> 'C'
>
> Estimated Cost: 546
> Estimated # of Rows Returned: 1
> Maximum Threads: 0
>
> 1) devdba.v_inq: INDEX PATH
>
> Filters: devdba.v_inq.vg_stat != 'C'
>
> (1) Index Name: devdba.vi_vid_idx
>
> Index Keys: v_id (Parallel, fragments: ALL)
>
> Lower Index Filter: devdba.v_inq.v_id = ANY <subquery>
>
> Subquery:
>
> ---------
>
> Estimated Cost: 122589
>
> Estimated # of Rows Returned: 1
>
> Maximum Threads: 1
>
> 1) devdba.s1: INDEX PATH
>
> (1) Index Name: mthly_info_idx1
>
> Index Keys: v_id (Key-First) (Parallel, fragments: ALL)
>
> Index Key Filters: (devdba.s1.v_id = TO_CHAR (devdba.v_inq.v_id ) )
>
> Subquery:
>
> ---------
>
> Estimated Cost: 544
>
> Estimated # of Rows Returned: 1
>
> Maximum Threads: 5
>
> 1) devdba.m: SEQUENTIAL SCAN
>
> Filters: (devdba.m.cit = 'U' AND devdba.m.typ = 'E' )
>
> 2) devdba.s: INDEX PATH
>
> Filters: devdba.s.status = 'C'
>
> (1) Index Name: mthly_info_idx1
>
> Index Keys: v_id (Parallel, fragments: ALL)
>
> Lower Index Filter: devdba.m.v_id = devdba.s.v_id NESTED LOOP JOIN
>
> Query statistics:
> -----------------
>
> Table map :
> ----------------------------
> Internal name Table name
> ----------------------------
> t1 voter_inquiry
>
> type table rows_prod est_rows rows_scan time est_cost
> -------------------------------------------------------------------
> scan t1 179 1 188 00:00.24 547
>
> Subquery statistics:
> --------------------
>
> Table map :
> ----------------------------
> Internal name Table name
> ----------------------------
> t1 m
> t2 s
>
> type table rows_prod est_rows rows_scan time est_cost
> -------------------------------------------------------------------
> scan t1 997 94 9365 00:00.01 411
>
> type table rows_prod est_rows rows_scan time est_cost
> -------------------------------------------------------------------
> scan t2 188 275859 997 00:00.25 1
>
> type rows_prod est_rows time est_cost
> -------------------------------------------------
> nljoin 188 9 00:00.32 545
>
> type rows_sort est_rows rows_cons time
> -------------------------------------------------
> sort 183 1 188 00:00.10
>
> Subquery statistics:
> --------------------
>
> Table map :
> ----------------------------
> Internal name Table name
> ----------------------------
> t1 s1
>
> type table rows_prod est_rows rows_scan time est_cost
> -------------------------------------------------------------------
> scan t1 16110 1 49808753010 10155:54.95 122590
>
> type rows_sort est_rows rows_cons time
> -------------------------------------------------
> sort 179 1 179 113:03.41
>
>
> ***********************************************************************
> ********
> Forum Note: Use "Reply" to post a response in the discussion forum.

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