Hi folks.

I have the query below which when run takes approx 3 seconds. However when I
add the condition 'and w_ws_id = 10' onto the end changes to 30+ seconds.

Can anyone see why? I've included the explain for the long select.

select w_vin as key, w_vin, v_o_number as vista, v_status,
s_stock_no, s_customer_order,
coalesce(d.r_registration, p.r_registration, w_reg_no) as
reg_no,w_arrival,
w_updated::date,CURRENT_DATE-w_created::date as age, w_model,
w_radio_code, w_key_no, w_inspected, w_damage,
w_walon_repair,w_collect_date, w_despatch_date, w_sheet,
w_plates, w_accessories,
coalesce(p.d_des, d.d_des) as d_des,
coalesce(p.de_des, d.de_des) as de_des,
p.r_id as pdi,
d.r_id as delivery,
o.o_id, o.state
from walon
left outer join request_details p on p.t_id = 'P' and
substring(p.r_chassis from '(.{11}$)') = w_vin
left outer join request_details d on d.t_id = 'D' and
substring(d.r_chassis from '(.{11}$)') = w_vin
left outer join order_details o on
substring(o.o_vin from '(.{11}$)') = w_vin
left outer join stock s on substring(s.s_vin from '(.{11}$)') = w_vin
left outer join vista v on v.v_vin = w_vin
where w_hide = 0.


--
Gary Stainburn

This email does not contain private or confidential material as it
may be snooped on by interested government parties for unknown
and undisclosed purposes - Regulation of Investigatory Powers Act, 2000


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Gary Stainburn <gary.stainburn@ringways.co.uk> writes:
> I have the query below which when run takes approx 3 seconds. However when I
> add the condition 'and w_ws_id = 10' onto the end changes to 30+ seconds.
> Can anyone see why? I've included the explain for the long select.

You really ought to provide EXPLAIN ANALYZE output for both versions if
you want intelligent commentary --- otherwise we're just guessing.

But I would guess the problem is that the planner is way off about the
number of rows satisfying the joint condition --- it thinks two:

> -> Index Scan using walon_hide_index on walon (cost=0.00..85.04 rows=2 width=140)
> Index Cond: (w_hide = 0)
> Filter: (w_ws_id = 10)

This plan is good if there really are only a couple rows, but degrades
quickly if there are many...

regards, tom lane

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On Saturday 05 May 2007 01:57, Tom Lane wrote:
> Gary Stainburn <gary.stainburn@ringways.co.uk> writes:
> > I have the query below which when run takes approx 3 seconds. However
> > when I add the condition 'and w_ws_id = 10' onto the end changes to 30+
> > seconds. Can anyone see why? I've included the explain for the long
> > select.
>
> You really ought to provide EXPLAIN ANALYZE output for both versions if
> you want intelligent commentary --- otherwise we're just guessing.
>
> But I would guess the problem is that the planner is way off about the
>
> number of rows satisfying the joint condition --- it thinks two:
> > -> Index Scan using walon_hide_index on walon
> > (cost=0.00..85.04 rows=2 width=140) Index Cond: (w_hide = 0)
> > Filter: (w_ws_id = 10)
>
> This plan is good if there really are only a couple rows, but degrades
> quickly if there are many...
>
> regards, tom lane

Thanks Tom

I'll bear that in mind in future.

The problem was fixed by running a 'vacuum analyse' so it looks like the
planner was getting confused.

Gary


--
Gary Stainburn

This email does not contain private or confidential material as it
may be snooped on by interested government parties for unknown
and undisclosed purposes - Regulation of Investigatory Powers Act, 2000

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