问题
I am trying to debug a query in PostgreSQL that I've built to bucket market data in time buckets in arbitrary time intervals. Here is my table definition:
CREATE TABLE historical_ohlcv (
exchange_symbol TEXT NOT NULL,
symbol_id TEXT NOT NULL,
kafka_key TEXT NOT NULL,
open NUMERIC,
high NUMERIC,
low NUMERIC,
close NUMERIC,
volume NUMERIC,
time_open TIMESTAMP WITH TIME ZONE NOT NULL,
time_close TIMESTAMP WITH TIME ZONE,
CONSTRAINT historical_ohlcv_pkey
PRIMARY KEY (exchange_symbol, symbol_id, time_open)
);
CREATE INDEX symbol_id_idx
ON historical_ohlcv (symbol_id);
CREATE INDEX open_close_symbol_id
ON historical_ohlcv (time_open, time_close, exchange_symbol, symbol_id);
CREATE INDEX time_open_idx
ON historical_ohlcv (time_open);
CREATE INDEX time_close_idx
ON historical_ohlcv (time_close);
The table has ~25m rows currently. My query as an example for 1 hour, but could be 5 mins, 10 mins, 2 days, etc.
EXPLAIN ANALYZE WITH vals AS (
SELECT
NOW() - '5 months' :: INTERVAL AS frame_start,
NOW() AS frame_end,
INTERVAL '1 hour' AS t_interval
)
, grid AS (
SELECT
start_time,
lead(start_time, 1)
OVER (
ORDER BY start_time ) AS end_time
FROM (
SELECT
generate_series(frame_start, frame_end,
t_interval) AS start_time,
frame_end
FROM vals
) AS x
)
SELECT max(high)
FROM grid g
LEFT JOIN historical_ohlcv ohlcv ON ohlcv.time_open >= g.start_time
WHERE exchange_symbol = 'BINANCE'
AND symbol_id = 'ETHBTC'
GROUP BY start_time;
The WHERE clause could be any valid value in the table.
This technique was inspired by:
- Best way to count records by arbitrary time intervals in Rails+Postgres.
The idea is to make a common table and left join your data with that to indicate which bucket stuff is in. This query is really slow! It's currently taking 15s. Based on the query planner, we have a really expensive nested loop:
QUERY PLAN
HashAggregate (cost=2758432.05..2758434.05 rows=200 width=40) (actual time=16023.713..16023.817 rows=542 loops=1)
Group Key: g.start_time
CTE vals
-> Result (cost=0.00..0.02 rows=1 width=32) (actual time=0.005..0.005 rows=1 loops=1)
CTE grid
-> WindowAgg (cost=64.86..82.36 rows=1000 width=16) (actual time=2.986..9.594 rows=3625 loops=1)
-> Sort (cost=64.86..67.36 rows=1000 width=8) (actual time=2.981..4.014 rows=3625 loops=1)
Sort Key: x.start_time
Sort Method: quicksort Memory: 266kB
-> Subquery Scan on x (cost=0.00..15.03 rows=1000 width=8) (actual time=0.014..1.991 rows=3625 loops=1)
-> ProjectSet (cost=0.00..5.03 rows=1000 width=16) (actual time=0.013..1.048 rows=3625 loops=1)
-> CTE Scan on vals (cost=0.00..0.02 rows=1 width=32) (actual time=0.008..0.009 rows=1 loops=1)
-> Nested Loop (cost=0.56..2694021.34 rows=12865667 width=14) (actual time=7051.730..16015.873 rows=31978 loops=1)
-> CTE Scan on grid g (cost=0.00..20.00 rows=1000 width=16) (actual time=2.988..11.635 rows=3625 loops=1)
-> Index Scan using historical_ohlcv_pkey on historical_ohlcv ohlcv (cost=0.56..2565.34 rows=12866 width=22) (actual time=3.712..4.413 rows=9 loops=3625)
Index Cond: ((exchange_symbol = 'BINANCE'::text) AND (symbol_id = 'ETHBTC'::text) AND (time_open >= g.start_time))
Filter: (time_close < g.end_time)
Rows Removed by Filter: 15502
Planning time: 0.568 ms
Execution time: 16023.979 ms
My guess is this line is doing a lot:
LEFT JOIN historical_ohlcv ohlcv ON ohlcv.time_open >= g.start_time
AND ohlcv.time_close < g.end_time
But I'm not sure how to accomplish this in another way.
P.S. apologies if this belongs to dba.SE. I read the FAQ and this seemed too basic for that site, so I posted here.
Edits as requested:
SELECT avg(pg_column_size(t)) FROM historical_ohlcv t TABLESAMPLE SYSTEM (0.1); returns 107.632
For exchange_symbol, there are 3 unique values, for symbol_id there are ~400
PostgreSQL version: PostgreSQL 10.3 (Ubuntu 10.3-1.pgdg16.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609, 64-bit.
The table will be growing about ~1m records a day, so not exactly read-only. All this stuff is done locally and I will try to move to RDS or to help manage hardware issues.
Related: if I wanted to add other aggregates, specifically 'first in the bucket', 'last in the bucket', min, sum, would my indexing strategy change?
回答1:
Correctness first: I suspect a bug in your query:
LEFT JOIN historical_ohlcv ohlcv ON ohlcv.time_open >= g.start_time
AND ohlcv.time_close < g.end_time
Unlike my referenced answer, you join on a time interval: (time_open, time_close]. The way you do it excludes rows in the table where the interval crosses bucket borders. Only intervals fully contained in a single bucket count. I don't think that's intended?
A simple fix would be to decide bucket membership based on time_open (or time_close) alone. If you want to keep working with both, you have to define exactly how to deal with intervals overlapping with multiple buckets.
Also, you are looking for max(high) per bucket, which is different in nature from count(*) in my referenced answer.
And your buckets are simple intervals per hour?
Then we can radically simplify. Working with just time_open:
SELECT date_trunc('hour', time_open) AS hour, max(high) AS max_high
FROM historical_ohlcv
WHERE exchange_symbol = 'BINANCE'
AND symbol_id = 'ETHBTC'
AND time_open >= now() - interval '5 months' -- frame_start
AND time_open < now() -- frame_end
GROUP BY 1
ORDER BY 1;
Related:
- Resample on time series data
It's hard to talk about further performance optimization while basics are unclear. And we'd need more information.
Are WHERE conditions variable?
How many distinct values in exchange_symbol and symbol_id?
Avg. row size? What do you get for:
SELECT avg(pg_column_size(t)) FROM historical_ohlcv t TABLESAMPLE SYSTEM (0.1);
Is the table read-only?
Assuming you always filter on exchange_symbol and symbol_id and values are variable, your table is read-only or autovacuum can keep up with the write load so we can hope for index-only scans, you would best have a multicolumn index on (exchange_symbol, symbol_id, time_open, high DESC) to support this query. Index columns in this order. Related:
- Multicolumn index and performance
Depending on data distribution and other details a LEFT JOIN LATERAL solution might be another option. Related:
- How to find an average of values for time intervals in postgres
- Optimize GROUP BY query to retrieve latest record per user
Aside from all that, you EXPLAIN plan exhibits some very bad estimates:
- https://explain.depesz.com/s/E5yI
Are you using a current version of Postgres? You may have to work on your server configuration - or at least set higher statistics targets on relevant columns and more aggressive autovacuum settings for the big table. Related:
- Keep PostgreSQL from sometimes choosing a bad query plan
- Aggressive Autovacuum on PostgreSQL
来源：https://stackoverflow.com/questions/50221842/slow-left-join-on-cte-with-time-intervals