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31e1a9ce46
This patch implements SkipScan; an optimization for SELECT DISTINCT ON. Usually for SELECT DISTINCT ON postgres will plan either a UNIQUE over a sorted path, or some form of aggregate. In either case, it needs to scan the entire table, even in cases where there are only a few unique values. A skip scan optimizes this case when we have an ordered index. Instead of scanning the entire table and deduplicating after, the scan remembers the last value returned, and searches the index for the next value after that one. This means that for a table with k keys, with u distinct values, a skip scan runs in time u * log(k) as opposed to scanning then deduplicating, which takes time k. We can write the number of unique values u as of function of k by dividing by the number of repeats r i.e. u = k/r this means that a skip scan will be faster if each key is repeated more than a logarithmic number of times, i.e. if r > log(k) then u * log(k) < k/log(k) * log(k) < k. Co-authored-by: Joshua Lockerman <josh@timescale.com>
52 lines
3.3 KiB
PL/PgSQL
52 lines
3.3 KiB
PL/PgSQL
-- This file and its contents are licensed under the Timescale License.
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-- Please see the included NOTICE for copyright information and
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-- LICENSE-TIMESCALE for a copy of the license.
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CREATE TABLE skip_scan(time int, dev int, dev_name text, val int);
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INSERT INTO skip_scan SELECT t, d, 'device_' || d::text, t*d FROM generate_series(1, 1000) t, generate_series(1, 10) d;
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INSERT INTO skip_scan VALUES (NULL, 0, -1, NULL), (0, NULL, -1, NULL);
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INSERT INTO skip_scan(time,dev,dev_name,val) SELECT t, NULL, NULL, NULL FROM generate_series(0, 999, 50) t;
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ANALYZE skip_scan;
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CREATE TABLE skip_scan_nulls(time int);
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CREATE INDEX ON skip_scan_nulls(time);
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INSERT INTO skip_scan_nulls SELECT NULL FROM generate_series(1,100);
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ANALYZE skip_scan_nulls;
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-- create hypertable with different physical layouts in the chunks
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CREATE TABLE skip_scan_ht(f1 int, f2 int, f3 int, time int NOT NULL, dev int, dev_name text, val int);
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SELECT create_hypertable('skip_scan_ht', 'time', chunk_time_interval => 250, create_default_indexes => false);
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INSERT INTO skip_scan_ht(time,dev,dev_name,val) SELECT t, d, 'device_' || d::text, random() FROM generate_series(0, 249) t, generate_series(1, 10) d;
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ALTER TABLE skip_scan_ht DROP COLUMN f1;
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INSERT INTO skip_scan_ht(time,dev,dev_name,val) SELECT t, d, 'device_' || d::text, random() FROM generate_series(250, 499) t, generate_series(1, 10) d;
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ALTER TABLE skip_scan_ht DROP COLUMN f2;
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INSERT INTO skip_scan_ht(time,dev,dev_name,val) SELECT t, d, 'device_' || d::text, random() FROM generate_series(500, 749) t, generate_series(1, 10) d;
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ALTER TABLE skip_scan_ht DROP COLUMN f3;
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INSERT INTO skip_scan_ht(time,dev,dev_name,val) SELECT t, d, 'device_' || d::text, random() FROM generate_series(750, 999) t, generate_series(1, 10) d;
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INSERT INTO skip_scan_ht(time,dev,dev_name,val) SELECT t, NULL, NULL, NULL FROM generate_series(0, 999, 50) t;
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ANALYZE skip_scan_ht;
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ALTER TABLE skip_scan_ht SET (timescaledb.compress,timescaledb.compress_orderby='time desc', timescaledb.compress_segmentby='dev');
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CREATE TABLE skip_scan_insert(time int, dev int, dev_name text, val int, query text);
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CREATE OR REPLACE FUNCTION int_func_immutable() RETURNS int LANGUAGE SQL IMMUTABLE SECURITY DEFINER AS $$SELECT 1; $$;
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CREATE OR REPLACE FUNCTION int_func_stable() RETURNS int LANGUAGE SQL STABLE SECURITY DEFINER AS $$ SELECT 2; $$;
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CREATE OR REPLACE FUNCTION int_func_volatile() RETURNS int LANGUAGE SQL VOLATILE SECURITY DEFINER AS $$ SELECT 3; $$;
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CREATE OR REPLACE FUNCTION inta_func_immutable() RETURNS int[] LANGUAGE SQL IMMUTABLE SECURITY DEFINER AS $$ SELECT ARRAY[1,2,3]; $$;
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CREATE OR REPLACE FUNCTION inta_func_stable() RETURNS int[] LANGUAGE SQL STABLE SECURITY DEFINER AS $$ SELECT ARRAY[2,3,4]; $$;
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CREATE OR REPLACE FUNCTION inta_func_volatile() RETURNS int[] LANGUAGE SQL VOLATILE SECURITY DEFINER AS $$ SELECT ARRAY[3,4,5]; $$;
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-- adjust statistics so we get skipscan plans
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UPDATE pg_statistic SET stadistinct=1, stanullfrac=0.5 WHERE starelid='skip_scan'::regclass;
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UPDATE pg_statistic SET stadistinct=1, stanullfrac=0.5 WHERE starelid='skip_scan_nulls'::regclass;
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UPDATE pg_statistic SET stadistinct=1, stanullfrac=0.5 WHERE starelid='skip_scan_ht'::regclass;
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UPDATE pg_statistic SET stadistinct=1, stanullfrac=0.5 WHERE starelid IN (select inhrelid from pg_inherits where inhparent='skip_scan_ht'::regclass);
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