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IDS Forum

Poorly Optimized SQL

Posted By: MICHAEL HOFFMAN
Date: Tuesday, 6 August 2013, at 12:14 p.m.

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

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