Hi,
On 2023-04-16 19:00:33 +0200, Clemens Eisserer wrote:
> I am currently trying to migrate an influxdb 1.7 smarthome database to
> postgresql (13.9) running on my raspberry 3.
> It works quite well, but for the queries executed by grafana I get a
> bit highter execution times than I'd hoped for.
>
> Example:
> table smartmeter with non-null column ts (timestamp with time zone)
> and brinc index on ts, no pk to avoid a btree index.
> Sensor values are stored every 5s, so for 1 month there are about 370k
> rows - and in total the table currently holds about 3M rows.
> The query to display the values for 1 month takes ~3s, with the bitmap
> heap scan as well as aggregation taking up most of the time, with
> sorting in between.
>
> Is there anything that could be improved?
> With influxdb I was able to view 3 and 6 months graphs, with
> postgresql it simply takes too long.
>
> I am currently running the 32-bit ARMv6 build, would it be a big
> improvement running ARMv8/64-bit?
Yes, I suspect so. On a 64bit system most of the datatypes you're dealing with
are going to be pass-by-value, i.e. not incur memory allocation
overhead. Whereas timestamps, doubles, etc will all require allocations on a
32bit system.
> smarthomedb=> explain analyze SELECT floor(extract(epoch from
> ts)/10800)*10800 AS "time", AVG(stromL1) as l1, AVG(stromL2) as l2,
> AVG(stroml3) as l3 FROM smartmeter WHERE ts BETWEEN '2023-03-16
> T09:51:28.397Z' AND '2023-04-16T08:51:28.397Z' GROUP BY time order by time;
>
> QUERY PLAN
> ---------------------------------------------------------------------------------------------------------------------------------------------------------------------
> GroupAggregate (cost=117490.70..132536.10 rows=376135 width=32)
> (actual time=2061.253..2974.336 rows=236 loops=1)
> Group Key: ((floor((date_part('epoch'::text, ts) / '10800'::double
> precision)) * '10800'::double precision))
> -> Sort (cost=117490.70..118431.04 rows=376135 width=20) (actual
> time=2058.407..2410.467 rows=371810 loops=1)
> Sort Key: ((floor((date_part('epoch'::text, ts) /
> '10800'::double precision)) * '10800'::double precision))
Given the number of rows you're sorting on a somewhat slow platform, the
complexity of the expression here might be a relevant factor. Particularly on
a 32bit system (see above), due to the memory allocations we'll end up doing.
I don't know how much control over the query generation you have. Consider
rewriting
floor(extract(epoch from ts)/10800)*10800 AS "time"
to something like
date_bin('3h', ts, '2001-01-01 00:00')
> Sort Method: external merge Disk: 10960kB
> -> Bitmap Heap Scan on smartmeter (cost=112.09..74944.93
> rows=376135 width=20) (actual time=88.336..1377.862 rows=371810
> loops=1)
Given the time spent in the bitmap heap scan, it might be beneficial to
increase effective_io_concurrency some.
> Recheck Cond: ((ts >= '2023-03-16
> 10:51:28.397+01'::timestamp with time zone) AND (ts <= '2023-04-16
> 10:51:28.397+02'::timestamp with time zone))
> Rows Removed by Index Recheck: 2131
> Heap Blocks: lossy=4742
The lossiness might also incur some overhead, so increasing work_mem a bit
will help some.
Greetings,
Andres Freund
