我有一个数据库，可以获取大量的传感器数据（IOT之类） - 我将这些数据存储在一个名为events的表中。 （下面的结构）
现在，用户还可以订阅特定信号的变化（通过id），并且他们可以从特定点开始请求所有信号事件（只有他们订阅的信号）。
问题是这个查询真的很慢（1分钟+），我无法理解为什么这么慢。我错过了各种指数吗？
我想要执行的查询：
SELECT events.*
FROM Events
JOIN subscription_signal
ON subscription_signal.subscription_id = $1 AND
events.signal_id = subscription_signal.signal_id
WHERE source_time > $2
ORDER BY source_time ASC
LIMIT $3
create table if not exists events
(
source_time timestamp with time zone not null,
added_time timestamp with time zone default now(),
signal_id integer not null,
data_type text,
analog_value numeric(18,4) default 0,
digital_value boolean default false
);
create index if not exists events_source_time_idx
on new_events (source_time desc);
create index if not exists vents_signal_id_source_time_index
on new_events (signal_id, source_time);
create index if not exists events_source_time_signal_id_index
on new_events (source_time, signal_id);
create index if not exists events_signal_id_index
on new_events (signal_id);
create table if not exists subscription_signal
(
subscription_id integer not null,
signal_id integer not null,
constraint subscription_signal_subscription_id_signal_id_uindex
unique (subscription_id, signal_id)
);
我已经尝试添加多个索引，但似乎没有什么可以改善查询时间。
下面是使用EXPLAIN ANALYZE为11的subscription_id，在signal_id表中有114个subscription_signals。
EXPLAIN ANALYZE：
Limit (cost=0.71..0.71 rows=500 width=33) (actual time=24018.215..372251.399 rows=274 loops=1)
-> Nested Loop (cost=0.71..0.30 rows=72839547 width=33) (actual time=24018.214..372251.287 rows=274 loops=1)
-> Append (cost=0.43..0.00 rows=0 width=33) (actual time=0.038..141484.028 rows=133615131 loops=1)
-> Index Scan Backward using _hyper_1_2_chunk_events_source_timestamp_idx on _hyper_1_2_chunk (cost=0.43..184291.13 rows=5852043 width=32) (actual time=0.038..9359.166 rows=5852114 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_3_chunk_events_source_timestamp_idx on _hyper_1_3_chunk (cost=0.43..119619.56 rows=3733367 width=33) (actual time=0.096..4841.893 rows=3733367 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_4_chunk_events_source_timestamp_idx on _hyper_1_4_chunk (cost=0.43..101122.76 rows=3188104 width=33) (actual time=0.065..2088.177 rows=3188104 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_5_chunk_events_source_timestamp_idx on _hyper_1_5_chunk (cost=0.43..103410.32 rows=3223702 width=33) (actual time=0.055..3959.425 rows=3223702 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_6_chunk_events_source_timestamp_idx on _hyper_1_6_chunk (cost=0.43..130778.92 rows=4116765 width=33) (actual time=0.039..2917.270 rows=4116765 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_7_chunk_events_source_timestamp_idx on _hyper_1_7_chunk (cost=0.43..160960.05 rows=5047161 width=33) (actual time=0.030..3801.518 rows=5047161 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_8_chunk_events_source_timestamp_idx on _hyper_1_8_chunk (cost=0.43..161138.72 rows=5067119 width=33) (actual time=0.046..3106.090 rows=5067119 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_9_chunk_events_source_timestamp_idx on _hyper_1_9_chunk (cost=0.43..148498.73 rows=4523834 width=33) (actual time=0.050..4114.502 rows=4535789 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_10_chunk_events_source_timestamp_idx on _hyper_1_10_chunk (cost=0.43..149195.92 rows=4690233 width=33) (actual time=0.029..4904.397 rows=4690344 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_11_chunk_events_source_timestamp_idx on _hyper_1_11_chunk (cost=0.43..148274.33 rows=4627748 width=33) (actual time=0.034..4086.343 rows=4645264 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_12_chunk_events_source_timestamp_idx on _hyper_1_12_chunk (cost=0.43..147687.64 rows=4608886 width=33) (actual time=0.054..4305.309 rows=4612240 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_13_chunk_events_source_timestamp_idx on _hyper_1_13_chunk (cost=0.43..137341.97 rows=4280236 width=33) (actual time=0.049..6224.372 rows=4301055 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_14_chunk_events_source_timestamp_idx on _hyper_1_14_chunk (cost=0.43..141762.95 rows=4460189 width=33) (actual time=0.054..2724.318 rows=4460189 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_15_chunk_events_source_timestamp_idx on _hyper_1_15_chunk (cost=0.43..143731.09 rows=4522774 width=33) (actual time=0.029..3981.750 rows=4522875 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_16_chunk_events_source_timestamp_idx on _hyper_1_16_chunk (cost=0.43..150538.02 rows=4721239 width=33) (actual time=0.062..6109.135 rows=4722110 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_17_chunk_events_source_timestamp_idx on _hyper_1_17_chunk (cost=0.43..142216.56 rows=4475544 width=33) (actual time=0.060..3955.418 rows=4475511 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_18_chunk_events_source_timestamp_idx on _hyper_1_18_chunk (cost=0.43..137131.47 rows=4293396 width=33) (actual time=0.072..5002.332 rows=4294763 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_19_chunk_events_source_timestamp_idx on _hyper_1_19_chunk (cost=0.43..136908.18 rows=4284368 width=33) (actual time=0.100..5866.330 rows=4279079 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_20_chunk_events_source_timestamp_idx on _hyper_1_20_chunk (cost=0.43..142385.12 rows=4475205 width=33) (actual time=0.081..3407.288 rows=4475211 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_21_chunk_events_source_timestamp_idx on _hyper_1_21_chunk (cost=0.43..154334.53 rows=4854714 width=33) (actual time=0.082..3049.434 rows=4855427 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_22_chunk_events_source_timestamp_idx on _hyper_1_22_chunk (cost=0.43..149148.27 rows=4690356 width=33) (actual time=0.051..5394.662 rows=4690489 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_23_chunk_events_source_timestamp_idx on _hyper_1_23_chunk (cost=0.43..147018.61 rows=4624741 width=33) (actual time=0.042..2781.965 rows=4624838 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_24_chunk_events_source_timestamp_idx on _hyper_1_24_chunk (cost=0.43..151841.97 rows=4774145 width=33) (actual time=0.053..5188.953 rows=4776442 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_25_chunk_events_source_timestamp_idx on _hyper_1_25_chunk (cost=0.43..141470.65 rows=4449075 width=32) (actual time=0.066..4113.208 rows=4451125 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_26_chunk_events_source_timestamp_idx on _hyper_1_26_chunk (cost=0.43..133228.19 rows=4192152 width=32) (actual time=0.073..3794.901 rows=4192089 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_27_chunk_events_source_timestamp_idx on _hyper_1_27_chunk (cost=0.43..93412.39 rows=2946643 width=33) (actual time=0.034..2600.225 rows=2946741 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_28_chunk_events_source_timestamp_idx on _hyper_1_28_chunk (cost=0.43..107666.24 rows=3380023 width=33) (actual time=0.061..4010.668 rows=3380023 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_29_chunk_events_source_timestamp_idx on _hyper_1_29_chunk (cost=0.43..139070.65 rows=4376229 width=33) (actual time=0.058..3597.805 rows=4376329 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_30_chunk_events_source_timestamp_idx on _hyper_1_30_chunk (cost=0.43..133595.88 rows=4204134 width=33) (actual time=0.047..3998.751 rows=4204442 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_31_chunk_events_source_timestamp_idx on _hyper_1_31_chunk (cost=0.43..139554.54 rows=4391212 width=33) (actual time=0.054..4415.468 rows=4391126 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_32_chunk_events_source_timestamp_idx on _hyper_1_32_chunk (cost=0.43..78562.22 rows=2463045 width=33) (actual time=0.054..1672.220 rows=2482753 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_1_chunk_events_source_timestamp_idx on _hyper_1_1_chunk (cost=0.14..5.59 rows=62 width=32) (actual time=12.567..12.603 rows=62 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_65_chunk_events_source_timestamp_idx on _hyper_1_65_chunk (cost=0.15..16.92 rows=267 width=73) (actual time=8.766..8.768 rows=1 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_88_chunk_events_source_timestamp_idx on _hyper_1_88_chunk (cost=0.15..12.90 rows=364 width=32) (actual time=7.869..8.008 rows=374 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Scan Backward using _hyper_1_89_chunk_events_source_timestamp_idx on _hyper_1_89_chunk (cost=0.14..6.16 rows=94 width=32) (actual time=0.017..0.047 rows=108 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
-> Index Only Scan using subscription_signal_subscription_id_signal_id_uindex on subscription_signal (cost=0.28..0.30 rows=1 width=4) (actual time=0.001..0.001 rows=0 loops=133615131)
Index Cond: ((subscription_id = 11) AND (signal_id = _hyper_1_2_chunk.signal_id))
Heap Fetches: 274
Planning time: 8.924 ms
Execution time: 372252.454 ms
EXPLAIN (ANALYZE, BUFFERS)：
Limit (cost=0.71..0.71 rows=500 width=33) (actual time=19504.097..315933.258 rows=274 loops=1)
Buffers: shared hit=269362667 read=1352546
-> Nested Loop (cost=0.71..0.30 rows=72839557 width=33) (actual time=19504.096..315933.158 rows=274 loops=1)
Buffers: shared hit=269362667 read=1352546
-> Append (cost=0.43..0.00 rows=0 width=33) (actual time=14.950..96402.213 rows=133615135 loops=1)
Buffers: shared hit=2132123 read=1352546
-> Index Scan Backward using _hyper_1_2_chunk_events_source_timestamp_idx on _hyper_1_2_chunk (cost=0.43..184291.13 rows=5852043 width=32) (actual time=0.019..4707.774 rows=5852114 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=265328 read=20465
-> Index Scan Backward using _hyper_1_3_chunk_events_source_timestamp_idx on _hyper_1_3_chunk (cost=0.43..119619.56 rows=3733367 width=33) (actual time=0.057..2192.118 rows=3733367 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=72014 read=25960
-> Index Scan Backward using _hyper_1_4_chunk_events_source_timestamp_idx on _hyper_1_4_chunk (cost=0.43..101122.76 rows=3188104 width=33) (actual time=0.064..1964.159 rows=3188104 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=60389 read=23035
-> Index Scan Backward using _hyper_1_5_chunk_events_source_timestamp_idx on _hyper_1_5_chunk (cost=0.43..103410.32 rows=3223702 width=33) (actual time=0.033..1769.853 rows=3223702 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=61428 read=23456
-> Index Scan Backward using _hyper_1_6_chunk_events_source_timestamp_idx on _hyper_1_6_chunk (cost=0.43..130778.92 rows=4116765 width=33) (actual time=0.029..2138.480 rows=4116765 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=72968 read=30233
-> Index Scan Backward using _hyper_1_7_chunk_events_source_timestamp_idx on _hyper_1_7_chunk (cost=0.43..160960.05 rows=5047161 width=33) (actual time=0.029..2628.668 rows=5047161 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=93741 read=35257
-> Index Scan Backward using _hyper_1_8_chunk_events_source_timestamp_idx on _hyper_1_8_chunk (cost=0.43..161138.72 rows=5067119 width=33) (actual time=0.036..2523.536 rows=5067119 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=93406 read=35384
-> Index Scan Backward using _hyper_1_9_chunk_events_source_timestamp_idx on _hyper_1_9_chunk (cost=0.43..148498.73 rows=4523834 width=33) (actual time=0.024..2890.820 rows=4535789 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=86274 read=32252
-> Index Scan Backward using _hyper_1_10_chunk_events_source_timestamp_idx on _hyper_1_10_chunk (cost=0.43..149195.92 rows=4690233 width=33) (actual time=0.025..2765.170 rows=4690344 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=84883 read=37839
-> Index Scan Backward using _hyper_1_11_chunk_events_source_timestamp_idx on _hyper_1_11_chunk (cost=0.43..148274.33 rows=4627748 width=33) (actual time=0.027..2902.346 rows=4645264 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=64046 read=59908
-> Index Scan Backward using _hyper_1_12_chunk_events_source_timestamp_idx on _hyper_1_12_chunk (cost=0.43..147687.64 rows=4608886 width=33) (actual time=0.049..3556.359 rows=4612240 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=63025 read=59693
-> Index Scan Backward using _hyper_1_13_chunk_events_source_timestamp_idx on _hyper_1_13_chunk (cost=0.43..137341.97 rows=4280236 width=33) (actual time=0.027..3361.873 rows=4301055 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=54986 read=55571
-> Index Scan Backward using _hyper_1_14_chunk_events_source_timestamp_idx on _hyper_1_14_chunk (cost=0.43..141762.95 rows=4460189 width=33) (actual time=0.028..2859.309 rows=4460189 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=50767 read=57846
-> Index Scan Backward using _hyper_1_15_chunk_events_source_timestamp_idx on _hyper_1_15_chunk (cost=0.43..143731.09 rows=4522774 width=33) (actual time=0.028..3143.475 rows=4522875 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=54841 read=58390
-> Index Scan Backward using _hyper_1_16_chunk_events_source_timestamp_idx on _hyper_1_16_chunk (cost=0.43..150538.02 rows=4721239 width=33) (actual time=0.028..2854.384 rows=4722110 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=56488 read=61480
-> Index Scan Backward using _hyper_1_17_chunk_events_source_timestamp_idx on _hyper_1_17_chunk (cost=0.43..142216.56 rows=4475544 width=33) (actual time=0.060..2522.463 rows=4475511 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=54403 read=57798
-> Index Scan Backward using _hyper_1_18_chunk_events_source_timestamp_idx on _hyper_1_18_chunk (cost=0.43..137131.47 rows=4293396 width=33) (actual time=0.051..2854.544 rows=4294763 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=49427 read=56050
-> Index Scan Backward using _hyper_1_19_chunk_events_source_timestamp_idx on _hyper_1_19_chunk (cost=0.43..136908.18 rows=4284368 width=33) (actual time=0.033..2673.792 rows=4279079 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=49055 read=55966
-> Index Scan Backward using _hyper_1_20_chunk_events_source_timestamp_idx on _hyper_1_20_chunk (cost=0.43..142385.12 rows=4475205 width=33) (actual time=0.029..2793.156 rows=4475211 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=60428 read=57064
-> Index Scan Backward using _hyper_1_21_chunk_events_source_timestamp_idx on _hyper_1_21_chunk (cost=0.43..154334.53 rows=4854714 width=33) (actual time=0.022..2701.886 rows=4855427 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=63812 read=62451
-> Index Scan Backward using _hyper_1_22_chunk_events_source_timestamp_idx on _hyper_1_22_chunk (cost=0.43..149148.27 rows=4690356 width=33) (actual time=0.028..2763.383 rows=4690489 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=62684 read=60247
-> Index Scan Backward using _hyper_1_23_chunk_events_source_timestamp_idx on _hyper_1_23_chunk (cost=0.43..147018.61 rows=4624741 width=33) (actual time=0.025..2823.549 rows=4624838 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=52489 read=59932
-> Index Scan Backward using _hyper_1_24_chunk_events_source_timestamp_idx on _hyper_1_24_chunk (cost=0.43..151841.97 rows=4774145 width=33) (actual time=0.060..2819.668 rows=4776442 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=48917 read=62622
-> Index Scan Backward using _hyper_1_25_chunk_events_source_timestamp_idx on _hyper_1_25_chunk (cost=0.43..141470.65 rows=4449075 width=32) (actual time=0.057..2554.301 rows=4451125 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=40129 read=58563
-> Index Scan Backward using _hyper_1_26_chunk_events_source_timestamp_idx on _hyper_1_26_chunk (cost=0.43..133228.19 rows=4192152 width=32) (actual time=0.049..2467.922 rows=4192089 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=55849 read=37756
-> Index Scan Backward using _hyper_1_27_chunk_events_source_timestamp_idx on _hyper_1_27_chunk (cost=0.43..93412.39 rows=2946643 width=33) (actual time=0.048..1754.935 rows=2946741 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=47717 read=23505
-> Index Scan Backward using _hyper_1_28_chunk_events_source_timestamp_idx on _hyper_1_28_chunk (cost=0.43..107666.24 rows=3380023 width=33) (actual time=0.027..1896.157 rows=3380023 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=66850 read=23284
-> Index Scan Backward using _hyper_1_29_chunk_events_source_timestamp_idx on _hyper_1_29_chunk (cost=0.43..139070.65 rows=4376229 width=33) (actual time=0.028..2469.809 rows=4376329 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=77118 read=31953
-> Index Scan Backward using _hyper_1_30_chunk_events_source_timestamp_idx on _hyper_1_30_chunk (cost=0.43..133595.88 rows=4204134 width=33) (actual time=0.032..2862.879 rows=4204442 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=64898 read=33521
-> Index Scan Backward using _hyper_1_31_chunk_events_source_timestamp_idx on _hyper_1_31_chunk (cost=0.43..139554.54 rows=4391212 width=33) (actual time=0.038..2431.625 rows=4391126 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=68406 read=34357
-> Index Scan Backward using _hyper_1_32_chunk_events_source_timestamp_idx on _hyper_1_32_chunk (cost=0.43..78562.22 rows=2463045 width=33) (actual time=0.060..1396.815 rows=2482753 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=35270 read=20706
-> Index Scan Backward using _hyper_1_1_chunk_events_source_timestamp_idx on _hyper_1_1_chunk (cost=0.14..5.59 rows=62 width=32) (actual time=0.030..0.095 rows=62 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=4 read=1
-> Index Scan Backward using _hyper_1_65_chunk_events_source_timestamp_idx on _hyper_1_65_chunk (cost=0.15..16.92 rows=267 width=73) (actual time=0.042..0.043 rows=1 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=2 read=1
-> Index Scan Backward using _hyper_1_88_chunk_events_source_timestamp_idx on _hyper_1_88_chunk (cost=0.15..12.90 rows=364 width=32) (actual time=0.022..0.140 rows=374 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=72
-> Index Scan Backward using _hyper_1_89_chunk_events_source_timestamp_idx on _hyper_1_89_chunk (cost=0.14..6.48 rows=112 width=32) (actual time=0.019..0.053 rows=112 loops=1)
Index Cond: (source_timestamp > '2018-07-05 09:00:00.88+00'::timestamp with time zone)
Buffers: shared hit=9
-> Index Only Scan using subscription_signal_subscription_id_signal_id_uindex on subscription_signal (cost=0.28..0.30 rows=1 width=4) (actual time=0.001..0.001 rows=0 loops=133615135)
Index Cond: ((subscription_id = 11) AND (signal_id = _hyper_1_2_chunk.signal_id))
Heap Fetches: 274
Buffers: shared hit=267230544
Planning time: 144.220 ms
Execution time: 315933.896 ms
events没有被source_timestamp分区，对吧？
PostgreSQL别无选择，只能用source_timestamp > '2018-07-05 09:00:00.88+00'找到所有行，然后用events加入结果。这永远不会很快。
如果省略LIMIT使查询更快，你可以通过将PostgreSQL推向不使用嵌套循环连接来获得一些东西，但考虑到所涉及的行数，这似乎不太可能。
如果表由source_timestamp分区，则查询可能会快得多。
尝试删除除(signal_id, source_time)上的索引之外的所有索引并使用以下查询：
SELECT *
FROM events
WHERE signal_id IN (
SELECT signal_id
FROM subscription_signal
WHERE subscription_id = $1)
AND source_time > $2
ORDER BY source_time
LIMIT $3
然后postgresql应该使用剩余的索引来快速查找给定signal_id列表的行，然后使用索引的第二部分来查找带有source_time > $2的行。
在以下情况下，查询可能会缓慢运行：
iops和读取带宽的限制。我不知道你是否已经解决了你的问题但是当我遇到这个问题时，你可能想要自己查看解释输出：https://explain.depesz.com/s/4GYT
我不确切知道为什么（还），但是查询规划器应该生成一个MergeAppend而不是Append执行节点，以便能够将LIMIT压缩到每个分区（索引）扫描中，就像在我的测试设置中一样：https://explain.depesz.com/s/Fi9C
为了比较，同样的查询没有限制，显示我在这里使用的测试数据量：https://explain.depesz.com/s/AJ6（是的，只有37M中间行，但它显示效果）
对于不耐烦的数字：
Without LIMIT: 49,174.946 milliseconds
LIMIT 274 OFFSET 100000: 35,022.269 milliseconds
LIMIT 274: 2.710 milliseconds
（LIMIT的不同值以及查询参数而不是常量不会导致任何不同的计划）
这个MergeAppend的优化已经在PostgreSQL中发布了9.1版本（参见commit 6fbc323），有或没有Timescale使用分区但不存在简单的Append，这告诉我这是这里的关键瓶颈。这反过来可能会受到不是最新统计数据的影响（你在两个表上都做过analyze，或者？）。
即将推出的版本PostgreSQL 12也应该使用commit LIMIT ... OFFSET ...将Append压缩到959d00e执行节点。
OTOH，即使在具有20亿行的Timescale优化表中，每个分区有超过100万行，时间也可以用在I / O上的大部分中。要测试它，请执行以下操作并重新创建EXPLAIN (ANALYZE, BUFFERS, COSTS OFF)计划：
SET track_io_timing TO ON;
然后包括执行计划中每个相关节点的I / O操作的时间（总和），例如：
Limit (actual time=1546.517..1546.572 rows=274 loops=1)
Buffers: shared hit=73438 read=23452
I/O Timings: read=1316.373
-> Sort (actual time=1546.512..1546.536 rows=274 loops=1)
Sort Key: e.source_time
Sort Method: top-N heapsort Memory: 46kB
Buffers: shared hit=73438 read=23452
I/O Timings: read=1316.373
-> Nested Loop (actual time=7.147..1524.251 rows=69293 loops=1)
Buffers: shared hit=73438 read=23452
I/O Timings: read=1316.373
-> Index Only Scan using subscription_signal_subscription_id_signal_id_uindex on subscription_signal (actual time=0.038..0.236 rows=114 loops=1)
Index Cond: (subscription_id = $1)
Heap Fetches: 114
Buffers: shared hit=4
-> Append (actual time=0.334..13.184 rows=608 loops=114)
Buffers: shared hit=73434 read=23452
I/O Timings: read=1316.373
-> Seq Scan on events e (actual time=0.000..0.000 rows=0 loops=114)
Filter: ((source_time > $2) AND (subscription_signal.signal_id = signal_id))
-> Index Scan using _hyper_5_13094_chunk_events_signal_id_source_time_index on _hyper_5_13094_chunk e_1 (actual time=0.014..0.014 rows=0 loops=114)
Index Cond: ((signal_id = subscription_signal.signal_id) AND (source_time > $2))
Buffers: shared hit=340 read=3
I/O Timings: read=0.851
-> Index Scan using _hyper_5_13095_chunk_events_signal_id_source_time_index on _hyper_5_13095_chunk e_2 (actual time=0.009..0.009 rows=0 loops=114)
Index Cond: ((signal_id = subscription_signal.signal_id) AND (source_time > $2))
Buffers: shared hit=335 read=7
I/O Timings: read=0.398
...
这对于检测查询是否实际上受CPU限制或I / O限制（哪一个是更大的瓶颈，因此是改善情况的最佳投资）非常有用。
减少中间结果行：
或者，最重要的部分是显着降低中间行数。我们可以看到您实际返回的行数是符合条件的行的0.000205％，我真的建议您重新考虑您的标准。
在时间序列查询中，您几乎从未使用下限进行过滤，而是提出一个合理的时间间隔，导致有足够的行在95％的情况下由LIMIT返回，而其他5％在那里没有足够的行，只需使用后续时间范围再次查询。
所以，只需尝试以下查询：
SELECT events.*
FROM Events
JOIN subscription_signal
ON subscription_signal.subscription_id = $1 AND
events.signal_id = subscription_signal.signal_id
WHERE source_time > $2 AND source_time <= ($2 + '1 week'::interval)
ORDER BY source_time ASC
LIMIT $3
仅此一点就可以减少查询时间。
尝试这样的事情：
SELECT *
FROM events
WHERE signal_id IN ( SELECT signal_id
FROM subscription_signal
WHERE subscription_id = $1
AND source_time > $2 )
ORDER BY source_time ASC
LIMIT $3