Execution plan does not use index

[Peter Coppens]

Hello,

Consider the following PostgreSQL 9.6.18 tables
- measurement_value: time series table  with a unique key on
(device_id,timestamp) columns and a number of columns with measurements.
Table contains a large number of rows (>150million)
- device table: with device properties (short_id joins to device_id in
measurement_value table)
- device_configuration table: extra device properties, including a timezone

This query works fine and quick (returns ±320K rows in 3.2 seconds when
explain is removed)

explain
  select d.short_id,mv.timestamp,mv.I64_01
  from device d, device_configuration dc, measurement_value mv,
pg_timezone_names tzn
  where mv.device_id=d.short_id and dc.device_id = d.id and dc.latest=true
and dc.timezone=tzn.name and
mv.timestamp > '2020-11-06'::timestamp and mv.timestamp <
'2020-11-07'::timestamp

==>
Hash Join  (cost=23.63..2156655.51 rows=1454320 width=20)
  Hash Cond: (dc.timezone = pg_timezone_names.name)
  ->  Nested Loop  (cost=1.13..2089933.38 rows=409070 width=34)
->  Nested Loop  (cost=0.56..7497.34 rows=615 width=18)
  ->  Index Scan using device_short_id_key on device d
 (cost=0.28..2423.90 rows=683 width=20)
  ->  Index Scan using device_configuration_device_latest_idx
on device_configuration dc  (cost=0.28..7.42 rows=1 width=30)
Index Cond: ((device_id = d.id) AND (latest = true))
Filter: latest

*->  Index Scan using measurement_values_pkey on measurement_value
mv  (cost=0.57..3375.60 rows=1047 width=20)  Index Cond:
((device_id = d.short_id) AND ("timestamp" > '2020-11-06
00:00:00'::timestamp without time zone) AND ("timestamp" < '2020-11-07
*00:00:00'::timestamp
without time zone))
  ->  Hash  (cost=10.00..10.00 rows=1000 width=32)
->  Function Scan on pg_timezone_names  (cost=0.00..10.00 rows=1000
width=32)


When adding the timezone offset to the where clause, the query becomes
extremely slow due to a full table scan of the measurement_value table
(±320K rows in 11 minutes)

(See  ->  Seq Scan on measurement_value mv  (cost=0.00..6308233.77
rows=169617977 width=20))

explain
  select d.short_id,mv.timestamp,mv.I64_01
  from device d, device_configuration dc, measurement_value mv,
pg_timezone_names tzn
  where mv.device_id=d.short_id and dc.device_id = d.id and dc.latest=true
and dc.timezone=tzn.name and
mv.timestamp > '2020-11-06'::timestamp *- tzn.utc_offset* and
mv.timestamp < '2020-11-07'::timestamp *- tzn.utc_offset*

==>
Hash Join  (cost=683.93..18226567.32 rows=60331762 width=20)
  Hash Cond: (mv.device_id = d.short_id)
  Join Filter: ((mv."timestamp" > ('2020-11-06 00:00:00'::timestamp without
time zone - pg_timezone_names.utc_offset)) AND (mv."timestamp" <
('2020-11-07 00:00:00'::timestamp without time zone -
pg_timezone_names.utc_offset)))
  ->  *Seq Scan on measurement_value mv  (cost=0.00..6308233.77
rows=169617977 width=20)*
  ->  Hash  (cost=656.61..656.61 rows=2186 width=20)
->  Hash Join  (cost=77.87..656.61 rows=2186 width=20)
  Hash Cond: (dc.timezone = pg_timezone_names.name)
  ->  Hash Join  (cost=55.37..533.83 rows=615 width=18)
Hash Cond: (dc.device_id = d.id)
->  Seq Scan on device_configuration dc
 (cost=0.00..470.01 rows=615 width=30)
  Filter: latest
->  Hash  (cost=46.83..46.83 rows=683 width=20)
  ->  Seq Scan on device d  (cost=0.00..46.83
rows=683 width=20)
  ->  Hash  (cost=10.00..10.00 rows=1000 width=48)
->  Function Scan on pg_timezone_names
 (cost=0.00..10.00 rows=1000 width=48)


Adding the tzn.utc_offset results in the fact that the execution plan no
longer considers to use the index on the measurement_value table. Is there
any way the SQL can be rewritten so that the index is used? Or any other
solution so that the query with the timezone offset returns in a comparable
time?

Many thanks!

Peter

Re: Execution plan does not use index

[Peter Coppens]

Michael

Many thanks for spending your time on this. Your alternative does not help 
unfortunately (see execution plan)

Still a sequential scan on the complete table. I have tried many alternatives 
and somehow whenever I add a column that is not in the index (I64_01) the 
optimizer decides not to use the index. If I remove that column, the index is 
used. I guess it estimates that the extra indirection from index pages to the 
row pages is more costly than scanning the 168M records. Pretty sure it’s not, 
but I cannot explain it to the stubborn thing :)

Btw, thanks for the >= tip (I was aware of it)

Wkr,

Peter


Hash Join  (cost=683.93..7270857.46 rows=458127 width=20)
  Hash Cond: (mv_inner.device_id = d.short_id)
  Join Filter: ((mv_inner."timestamp" > ('2020-11-06 00:00:00'::timestamp 
without time zone - pg_timezone_names.utc_offset)) AND (mv_inner."timestamp" < 
('2020-11-07 00:00:00'::timestamp without time zone - 
pg_timezone_names.utc_offset)))
  ->  Seq Scan on measurement_value mv_inner  (cost=0.00..7166797.33 
rows=1287989 width=1006)
Filter: (("timestamp" > '2020-11-05 00:00:00'::timestamp without time 
zone) AND ("timestamp" < '2020-11-08 00:00:00'::timestamp without time zone))
  ->  Hash  (cost=656.61..656.61 rows=2186 width=20)
->  Hash Join  (cost=77.87..656.61 rows=2186 width=20)
  Hash Cond: (dc.timezone = pg_timezone_names.name)
  ->  Hash Join  (cost=55.37..533.83 rows=615 width=18)
Hash Cond: (dc.device_id = d.id)
->  Seq Scan on device_configuration dc  (cost=0.00..470.01 
rows=615 width=30)
  Filter: latest
->  Hash  (cost=46.83..46.83 rows=683 width=20)
  ->  Seq Scan on device d  (cost=0.00..46.83 rows=683 
width=20)
  ->  Hash  (cost=10.00..10.00 rows=1000 width=48)
->  Function Scan on pg_timezone_names  (cost=0.00..10.00 
rows=1000 width=48)



> On 10 Nov 2020, at 01:15, Michael Lewis  wrote:
> 
> On Mon, Nov 9, 2020 at 1:11 PM Peter Coppens  <mailto:peter.copp...@datylon.com>> wrote:
> Adding the tzn.utc_offset results in the fact that the execution plan no 
> longer considers to use the index on the measurement_value table. Is there 
> any way the SQL can be rewritten so that the index is used? Or any other 
> solution so that the query with the timezone offset returns in a comparable 
> time?
> 
> I am not aware of a best practice to handle this. Your where condition on 
> mv.timestamp now depends on several joins to do a filtering that used to be a 
> static range that can be scanned into the index as a first node in the plan. 
> I have sometimes used a sub-query on a broader condition that allows the use 
> of the index, and then fully reducing the set later. Something like this-
> 
> select d.short_id,mv.timestamp,mv.I64_01
>   from device d, device_configuration dc, (
> select mv.*
>   from measurement_value AS mv_inner
>   where mv.timestamp > '2020-11-06'::timestamp - interval '1 day' and 
> mv.timestamp < '2020-11-07'::timestamp + interval '1 day'
> offset 0 /* to prevent in-lining the join to the outside set */
> ) mv, pg_timezone_names tzn
>   where mv.device_id=d.short_id and dc.device_id = d.id <http://d.id/> and 
> dc.latest=true and dc.timezone=tzn.name <http://tzn.name/> and
> mv.timestamp > '2020-11-06'::timestamp - tzn.utc_offset and 
> mv.timestamp < '2020-11-07'::timestamp - tzn.utc_offset
> 
> By the way, it seems a little odd to be exclusive on both the begin and end. 
> I'd usually expect timestamp >= start_date and timestamp < end_date + 
> interval '1 day' to fully capture a 24 hour period. Right now, you are 
> excluding any data that happens to have a timestamp value with .00 
> seconds (midnight exactly).

Re: Execution plan does not use index

[Peter Coppens]

Pavel

Tx for the tip. But given that if the I64_01 column is removed from the select 
list, the index is used I guess the cast is not likely to be the cause.

Like so

explain 
  select d.short_id,mv.timestamp --,mv.I64_01 
  from device d, device_configuration dc, measurement_value mv, 
pg_timezone_names tzn 
  where mv.device_id=d.short_id and dc.device_id = d.id and dc.latest=true and 
dc.timezone=tzn.name and 
mv.timestamp > '2020-11-06'::timestamp - tzn.utc_offset and 
mv.timestamp < '2020-11-07'::timestamp - tzn.utc_offset


==>

Nested Loop  (cost=1.13..6217004.08 rows=60425437 width=12)
  ->  Nested Loop  (cost=0.56..21334.84 rows=2186 width=20)
Join Filter: (dc.timezone = pg_timezone_names.name)
->  Nested Loop  (cost=0.56..7497.34 rows=615 width=18)
  ->  Index Scan using device_short_id_key on device d  
(cost=0.28..2423.90 rows=683 width=20)
  ->  Index Scan using device_configuration_device_latest_idx on 
device_configuration dc  (cost=0.28..7.42 rows=1 width=30)
Index Cond: ((device_id = d.id) AND (latest = true))
Filter: latest
->  Function Scan on pg_timezone_names  (cost=0.00..10.00 rows=1000 
width=48)
  ->  Index Only Scan using measurement_values_pkey on measurement_value mv  
(cost=0.57..2399.33 rows=43492 width=12)
Index Cond: ((device_id = d.short_id) AND ("timestamp" > ('2020-11-06 
00:00:00'::timestamp without time zone - pg_timezone_names.utc_offset)) AND 
("timestamp" < ('2020-11-07 00:00:00'::timestamp without time zone - 
pg_timezone_names.utc_offset)))


Peter

> On 10 Nov 2020, at 08:25, Pavel Stehule  wrote:
> 
> 
> 
> út 10. 11. 2020 v 8:18 odesílatel Peter Coppens  <mailto:peter.copp...@datylon.com>> napsal:
> Michael
> 
> Many thanks for spending your time on this. Your alternative does not help 
> unfortunately (see execution plan)
> 
> Still a sequential scan on the complete table. I have tried many alternatives 
> and somehow whenever I add a column that is not in the index (I64_01) the 
> optimizer decides not to use the index. If I remove that column, the index is 
> used. I guess it estimates that the extra indirection from index pages to the 
> row pages is more costly than scanning the 168M records. Pretty sure it’s 
> not, but I cannot explain it to the stubborn thing :)
> 
> Btw, thanks for the >= tip (I was aware of it)
> 
> Wkr,
> 
> Peter
> 
> 
> Hash Join  (cost=683.93..7270857.46 rows=458127 width=20)
>   Hash Cond: (mv_inner.device_id = d.short_id)
>   Join Filter: ((mv_inner."timestamp" > ('2020-11-06 00:00:00'::timestamp 
> without time zone - pg_timezone_names.utc_offset)) AND (mv_inner."timestamp" 
> < ('2020-11-07 00:00:00'::timestamp without time zone - 
> pg_timezone_names.utc_offset)))
>   ->  Seq Scan on measurement_value mv_inner  (cost=0.00..7166797.33 
> rows=1287989 width=1006)
> Filter: (("timestamp" > '2020-11-05 00:00:00'::timestamp without time 
> zone) AND ("timestamp" < '2020-11-08 00:00:00'::timestamp without time zone))
> 
> when you see cast in filter, then you should check type equality in 
> constraints. With some exception Postgres uses indexes only when filtered 
> value has same type like column type.
> 
> Maybe there is inconsistency between timestamp (with time zone), and 
> timestamp without time zone
> 
> Regards
> 
> Pavel
> 
> 
>   ->  Hash  (cost=656.61..656.61 rows=2186 width=20)
> ->  Hash Join  (cost=77.87..656.61 rows=2186 width=20)
>   Hash Cond: (dc.timezone = pg_timezone_names.name 
> <http://pg_timezone_names.name/>)
>   ->  Hash Join  (cost=55.37..533.83 rows=615 width=18)
> Hash Cond: (dc.device_id = d.id <http://d.id/>)
> ->  Seq Scan on device_configuration dc  
> (cost=0.00..470.01 rows=615 width=30)
>   Filter: latest
> ->  Hash  (cost=46.83..46.83 rows=683 width=20)
>   ->  Seq Scan on device d  (cost=0.00..46.83 
> rows=683 width=20)
>   ->  Hash  (cost=10.00..10.00 rows=1000 width=48)
> ->  Function Scan on pg_timezone_names  (cost=0.00..10.00 
> rows=1000 width=48)
> 
> 
> 
>> On 10 Nov 2020, at 01:15, Michael Lewis > <mailto:mle...@entrata.com>> wrote:
>> 
>> On Mon, Nov 9, 2020 at 1:11 PM Peter Coppens > <mailto:peter.copp...@datylon.com>> wrote:
>> Adding the tzn.utc_offset results in the fact that the execution plan no 
>> longer considers to u

Re: Execution plan does not use index

[Peter Coppens]

Triggered by Michael mentioning subqueries I ended up trying

explain
  select d.short_id,mv.timestamp ,mv.I64_01
  from device d, device_configuration dc, measurement_value mv
  where mv.device_id=d.short_id and dc.device_id = d.id and dc.latest=true and 
mv.timestamp > '2020-11-06'::timestamp - (select tzn.utc_offset from 
pg_timezone_names tzn where tzn.name=dc.timezone) and mv.timestamp < 
'2020-11-07'::timestamp - (select tzn.utc_offset from pg_timezone_names tzn 
where tzn.name=dc.timezone)

==>
Nested Loop  (cost=25.85..84540074.64 rows=16996885 width=20)
  ->  Seq Scan on device_configuration dc  (cost=0.00..470.01 rows=615 width=30)
Filter: latest
  ->  Nested Loop  (cost=25.85..137027.83 rows=43494 width=36)
->  Index Scan using device_pkey on device d  (cost=0.28..7.23 rows=1 
width=20)
  Index Cond: (id = dc.device_id)
->  Index Scan using measurement_values_pkey on measurement_value mv  
(cost=25.58..136585.66 rows=43494 width=20)
  Index Cond: ((device_id = d.short_id) AND ("timestamp" > 
('2020-11-06 00:00:00'::timestamp without time zone - (SubPlan 1))) AND 
("timestamp" < ('2020-11-07 00:00:00'::timestamp without time zone - (SubPlan 
2
  SubPlan 1
->  Function Scan on pg_timezone_names  (cost=0.00..12.50 
rows=5 width=16)
  Filter: (name = dc.timezone)
  SubPlan 2
->  Function Scan on pg_timezone_names pg_timezone_names_1  
(cost=0.00..12.50 rows=5 width=16)
  Filter: (name = dc.timezone)


Now returns the 320K in less than 5sec. 

I was till now convinced that correlated subqueries or joins are equivalent. I 
guess I was wrong :). Wonder how stable this plan will be though

Peter

> On 10 Nov 2020, at 09:06, Peter Coppens  wrote:
> 
> Pavel
> 
> Tx for the tip. But given that if the I64_01 column is removed from the 
> select list, the index is used I guess the cast is not likely to be the cause.
> 
> Like so
> 
> explain 
>   select d.short_id,mv.timestamp --,mv.I64_01 
>   from device d, device_configuration dc, measurement_value mv, 
> pg_timezone_names tzn 
>   where mv.device_id=d.short_id and dc.device_id = d.id <http://d.id/> and 
> dc.latest=true and dc.timezone=tzn.name and 
> mv.timestamp > '2020-11-06'::timestamp - tzn.utc_offset and 
> mv.timestamp < '2020-11-07'::timestamp - tzn.utc_offset
> 
> 
> ==>
> 
> Nested Loop  (cost=1.13..6217004.08 rows=60425437 width=12)
>   ->  Nested Loop  (cost=0.56..21334.84 rows=2186 width=20)
> Join Filter: (dc.timezone = pg_timezone_names.name)
> ->  Nested Loop  (cost=0.56..7497.34 rows=615 width=18)
>   ->  Index Scan using device_short_id_key on device d  
> (cost=0.28..2423.90 rows=683 width=20)
>   ->  Index Scan using device_configuration_device_latest_idx on 
> device_configuration dc  (cost=0.28..7.42 rows=1 width=30)
> Index Cond: ((device_id = d.id <http://d.id/>) AND 
> (latest = true))
> Filter: latest
> ->  Function Scan on pg_timezone_names  (cost=0.00..10.00 rows=1000 
> width=48)
>   ->  Index Only Scan using measurement_values_pkey on measurement_value mv  
> (cost=0.57..2399.33 rows=43492 width=12)
> Index Cond: ((device_id = d.short_id) AND ("timestamp" > ('2020-11-06 
> 00:00:00'::timestamp without time zone - pg_timezone_names.utc_offset)) AND 
> ("timestamp" < ('2020-11-07 00:00:00'::timestamp without time zone - 
> pg_timezone_names.utc_offset)))
> 
> 
> Peter
> 
>> On 10 Nov 2020, at 08:25, Pavel Stehule > <mailto:pavel.steh...@gmail.com>> wrote:
>> 
>> 
>> 
>> út 10. 11. 2020 v 8:18 odesílatel Peter Coppens > <mailto:peter.copp...@datylon.com>> napsal:
>> Michael
>> 
>> Many thanks for spending your time on this. Your alternative does not help 
>> unfortunately (see execution plan)
>> 
>> Still a sequential scan on the complete table. I have tried many 
>> alternatives and somehow whenever I add a column that is not in the index 
>> (I64_01) the optimizer decides not to use the index. If I remove that 
>> column, the index is used. I guess it estimates that the extra indirection 
>> from index pages to the row pages is more costly than scanning the 168M 
>> records. Pretty sure it’s not, but I cannot explain it to the stubborn thing 
>> :)
>> 
>> Btw, thanks for the >= tip (I was aware of it)
>> 
>> Wkr,
>> 
>> Peter
>> 
>> 
>> Hash Join  (cost=683.93..

Re: Execution plan does not use index

[Peter Coppens]

> 
> Curious, what is seq_page_cost and random_page_cost?

show seq_page_cost
->1

show random_page_cost
->4


> Any idea of your cache hits for indexes?

No, I am afraid not. It’s been a long time since I went that deep in the RDBMS 
behaviour (must have been Oracle 7.something  :) )


> If they are very high and/or you have SSD or similar fast storage, then maybe 
> random_page_cost should be 1.1-2 and not default 4 (assuming seq_page_cost is 
> still 1). The planner will be more likely to use an index scan if the 
> expected cost for scanning an index (random) is closer to a sequential read. 
> Sorry if this explanation is completely superfluous and you have already 
> configured this.

I played around with some of the settings (also with setting enable_seqscan to 
false). While some of the plans did no longer use a full table scan, the 
performance did not really work out when I tried. Tbh, I never got too deep in 
those plans to find out why that would be, as I am not too keen on changing 
these settings in the first place.

> 
> It would be interesting to see explain (analyze buffers) output so we can see 
> the actual counts for these nodes. I'm rather surprised that the query I 
> provided didn't use the timestamp index unless the inclusion of 3 days worth 
> of range meant that it was estimated to be too high a fraction of the table. 
> If you just execute only the subquery, is the index used by chance?
> 
Index is not used for the subquery

explain 
select mv_inner.*
  from measurement_value AS mv_inner
  where mv_inner.timestamp > '2020-11-06'::timestamp - interval '1 day' and 
mv_inner.timestamp < '2020-11-07'::timestamp + interval '1 day'
offset 0

==>

Seq Scan on measurement_value mv_inner (cost=0.00..7175777.00 rows=1219616 
width=1006)
  Filter: (("timestamp" > '2020-11-05 00:00:00'::timestamp without time zone) 
AND ("timestamp" < '2020-11-08 00:00:00'::timestamp without time zone))



> Top posting (reply with all previous copied below) is discouraged on these 
> lists. I think because it makes the archives rather messy. 

Will try to remember

Re: Execution plan does not use index

[Peter Coppens]

> Curious, how accurate is that row count of 1.2 million records for 3 days?


Not to bad actually

select count(mv_inner.*)
from measurement_value AS mv_inner
where mv_inner.timestamp > '2020-11-06'::timestamp - interval '1 day' and 
mv_inner.timestamp < '2020-11-07'::timestamp + interval '1 day'
offset 0

==> 1128736


> How many total records in the table?

±168 million

> If you disable sequential scan, does it choose the index and what cost does 
> it show?

It chooses the index, but apparently to create some intermediate structure that 
then later still needs to be joined on the device_id. Probably requires 
scanning all pages of the index, which might explain why the performance is 
still not ok

set enable_seqscan to false;
explain
select d.short_id,mv.timestamp,mv.I64_01
  from 
device d
, device_configuration dc
, (
  select mv_inner.*
from measurement_value AS mv_inner
where mv_inner.timestamp > '2020-11-06'::timestamp - interval '1 day' 
and mv_inner.timestamp < '2020-11-07'::timestamp + interval '1 day'
offset 0
  ) mv
, pg_timezone_names tzn
  where mv.device_id=d.short_id and dc.device_id = d.id and dc.latest=true and 
dc.timezone=tzn.name and
mv.timestamp > '2020-11-06'::timestamp - tzn.utc_offset and 
mv.timestamp < '2020-11-07'::timestamp - tzn.utc_offset

==>


Hash Join  (cost=6677594.18..9545649.57 rows=434126 width=20)
  Hash Cond: (mv_inner.device_id = d.short_id)
  Join Filter: ((mv_inner."timestamp" > ('2020-11-06 00:00:00'::timestamp 
without time zone - pg_timezone_names.utc_offset)) AND (mv_inner."timestamp" < 
('2020-11-07 00:00:00'::timestamp without time zone - 
pg_timezone_names.utc_offset)))
  ->  Bitmap Heap Scan on measurement_value mv_inner  
(cost=6676540.29..9446603.90 rows=1220458 width=1006)
Recheck Cond: (("timestamp" > '2020-11-05 00:00:00'::timestamp without 
time zone) AND ("timestamp" < '2020-11-08 00:00:00'::timestamp without time 
zone))
->  Bitmap Index Scan on measurement_values_pkey  
(cost=0.00..6676235.18 rows=1220458 width=0)
  Index Cond: (("timestamp" > '2020-11-05 00:00:00'::timestamp 
without time zone) AND ("timestamp" < '2020-11-08 00:00:00'::timestamp without 
time zone))
  ->  Hash  (cost=1026.55..1026.55 rows=2187 width=20)
->  Hash Join  (cost=471.95..1026.55 rows=2187 width=20)
  Hash Cond: (dc.timezone = pg_timezone_names.name)
  ->  Hash Join  (cost=449.45..903.76 rows=615 width=18)
Hash Cond: (dc.device_id = d.id)
->  Bitmap Heap Scan on device_configuration dc  
(cost=242.72..688.58 rows=615 width=30)
  Filter: latest
  ->  Bitmap Index Scan on 
device_configuration_device_latest_idx  (cost=0.00..242.57 rows=615 width=0)
Index Cond: (latest = true)
->  Hash  (cost=198.19..198.19 rows=683 width=20)
  ->  Index Scan using device_short_id_key on device d  
(cost=0.28..198.19 rows=683 width=20)
  ->  Hash  (cost=10.00..10.00 rows=1000 width=48)
->  Function Scan on pg_timezone_names  (cost=0.00..10.00 
rows=1000 width=48)

Re: Execution plan does not use index

[Peter Coppens]

> 
> Ahhh. You don't have a single column index on the timestamp value or a multi 
> column one with timestamp first. No wonder the subquery didn't help. My 
> apologies for not realizing that before. Thanks for satisfying my curiosity 
> why it didn't perform like it should. Certainly, that index may or may not be 
> worth creating and maintaining.
Indeed. Typical queries use both device id and a timestamp range. Only in some 
queries more than a few devices are needed.

> 
> Obviously the 3 days worth of data is also too high (just quick and safe). 
> Depending on what your server timezone vs the the most divergent timezone on 
> a device, that could be tightened up. Regardless. If the 5 seconds runtime 
> you got to with the correlated subquery on the where clause is sufficient, 
> then no need to continue I suppose.
Yeah, I am ok with the performance now. The overall query  where this was 
extracted from (and which had multiple full table scans) went from more than 2 
hours to 30 seconds. Given it’s a daily ‘async’  report that is sufficient.

> 
> It seems odd to me to not do any basic adjustment of random_page_cost though. 
> It isn't a magic number that the core team know to be perfect. It is a 
> baseline that is likely to be quite different for each use case and server 
> config. While there are no hard and fast rules and absolute right answers, it 
> seems prudent to at least follow the advice of the community and lower it a 
> ways if storage is ssd style and/or cache hits are quite high.
Ic. Well I don’t mine experimenting with it, and will certainly remember it 
next time. I guess I was demotivated because I read lot’s of warnings but these 
might have been about disabling sequential scans and not about page cost 
settings.

Re: Execution plan does not use index

[Peter Coppens]

> Good luck!

Tx! And tx for your support.

Re: Need another set of eyes on this

[Peter Coppens]

> On 25 Jan 2021, at 20:17, James B. Byrne  wrote:
> 
> I am recreating a test database with data provided for a series of future
> exercises.  Presently I am trying to understand why a simple insert statement
> is not working.  The user invoking this insert is 'postgres' and has superuser
> privileges.
> 
> The error is this:
> 
> postgres : STATEMENT:  INSERT INTO public.stock(
>   stock_num, manu_code, description, unit_price, unit, unit_descr)
>   VALUES (3, "SHM", "aluminum Bat", 180.00, "case", "12/case");
> postgres : ERROR:  42703: column "SHM" does not exist at character 105
> postgres : LOCATION:  errorMissingColumn, parse_relation.c:3194
> postgres : STATEMENT:  INSERT INTO public.stock(
>   stock_num, manu_code, description, unit_price, unit, unit_descr)
>   VALUES (3, "SHM", "aluminum bat", 180.00, "case", "12/case”)

> ;

Try single quotes

Re: Order by not working

[Peter Coppens]

Not sure how you select the household

> 
> The result is (only the first column is shown):
> 
> household_name
> 
> "Garcia"
> "Armstrong"
> "Armstrong"
> "Bauer"
> "Bauer"
> "Berst"
> "Berst"
> "Minch ()"
> "Berst"
> “Besel”


but unless you select from the resulting table using again an order by, the 
rows will be returned in an undetermined order. Such is the nature of the 
relational model - there is no order you can rely on when selecting, unless you 
specify it

Hth,

Peter