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https://github.com/timescale/timescaledb.git
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e0d63e915c
These tests show that non-aggregated queries that have an ORDER BY time DESC LIMIT x structure perform well. Postgres processes such queries by using a time DESC index and then performs a MergeAppend. Some further optimization to avoid touching unnecessary tables due to constraints can be added but they are not a priority. Much worse is that these optimizations do not work when grouping by time (either through integer division or using date_trunc). In order to use the non-aggregated query optimizations we had to create an index with time as the leading field and no WHERE clause on the index.
55 lines
2.6 KiB
SQL
55 lines
2.6 KiB
SQL
\set ON_ERROR_STOP 1
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\o /dev/null
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\ir include/insert_two_partitions.sql
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\o
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\set ECHO ALL
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\c single
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SELECT * FROM PUBLIC."testNs";
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EXPLAIN (verbose ON, costs off) SELECT * FROM PUBLIC."testNs";
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\echo "The following queries should NOT scan testNs._hyper_1_1_0_partition"
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EXPLAIN (verbose ON, costs off) SELECT * FROM PUBLIC."testNs" WHERE device_id = 'dev20';
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EXPLAIN (verbose ON, costs off) SELECT * FROM PUBLIC."testNs" WHERE device_id = 'dev'||'20';
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EXPLAIN (verbose ON, costs off) SELECT * FROM PUBLIC."testNs" WHERE 'dev'||'20' = device_id;
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--TODO: handle this later?
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--EXPLAIN (verbose ON, costs off) SELECT * FROM "testNs" WHERE device_id IN ('dev20', 'dev21');
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\echo "The following shows non-aggregated queries with time desc using merge append"
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EXPLAIN (verbose ON, costs off)SELECT * FROM PUBLIC."testNs" ORDER BY "timeCustom" DESC NULLS LAST limit 2;
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--shows that more specific indexes are used if the WHERE clauses "match", uses the series_1 index here.
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EXPLAIN (verbose ON, costs off)SELECT * FROM PUBLIC."testNs" WHERE series_1 IS NOT NULL ORDER BY "timeCustom" DESC NULLS LAST limit 2;
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--here the "match" is implication series_1 > 1 => series_1 IS NOT NULL
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EXPLAIN (verbose ON, costs off)SELECT * FROM PUBLIC."testNs" WHERE series_1 > 1 ORDER BY "timeCustom" DESC NULLS LAST limit 2;
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--note that without time transform things work too
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EXPLAIN (verbose ON, costs off)SELECT "timeCustom" t, min(series_0) FROM PUBLIC."testNs" GROUP BY t ORDER BY t DESC NULLS LAST limit 2;
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--TODO: time transform doesn't work
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EXPLAIN (verbose ON, costs off)SELECT "timeCustom"/10 t, min(series_0) FROM PUBLIC."testNs" GROUP BY t ORDER BY t DESC NULLS LAST limit 2;
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EXPLAIN (verbose ON, costs off)SELECT "timeCustom"%10 t, min(series_0) FROM PUBLIC."testNs" GROUP BY t ORDER BY t DESC NULLS LAST limit 2;
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--make table with timestamp. Test timestamp instead of int time.
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CREATE TABLE PUBLIC.hyper_1 (
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time TIMESTAMPTZ NOT NULL,
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series_0 DOUBLE PRECISION NULL,
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series_1 DOUBLE PRECISION NULL,
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series_2 DOUBLE PRECISION NULL
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);
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CREATE INDEX ON PUBLIC.hyper_1 (time DESC, series_0);
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SELECT * FROM create_hypertable('"public"."hyper_1"'::regclass, 'time'::name, number_partitions => 1, chunk_size_bytes=>100000);
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INSERT INTO hyper_1 SELECT to_timestamp(generate_series(0,10000)), random(), random(), random();
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--non-aggragated uses MergeAppend correctly
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EXPLAIN (verbose ON, costs off)SELECT * FROM hyper_1 ORDER BY "time" DESC limit 2;
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--TODO: aggregated with date_trunc doesn't work
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EXPLAIN (verbose ON, costs off)SELECT date_trunc('minute', time) t, min(series_0) FROM hyper_1 GROUP BY t ORDER BY t DESC limit 2;
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