Re: Execution plan does not use index

On 10 Nov 2020, at 09:06, Peter Coppens <peter.coppens@datylon.com> 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 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 <pavel.stehule@gmail.com> wrote:

út 10. 11. 2020 v 8:18 odesílatel Peter Coppens <peter.coppens@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)

              ->  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 <mlewis@entrata.com> wrote:

On Mon, Nov 9, 2020 at 1:11 PM Peter Coppens <peter.coppens@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 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

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 .000000 seconds (midnight exactly).