mirror of https://github.com/citusdata/citus.git
78 lines
2.5 KiB
SQL
78 lines
2.5 KiB
SQL
--
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-- MULTI_LIMIT_CLAUSE_APPROXIMATE
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--
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-- Display debug messages on limit clause push down.
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SET client_min_messages TO DEBUG1;
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-- We first look at results with limit optimization disabled. This first query
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-- has a group and an order by. The order by clause is a commutative aggregate
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-- function.
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SELECT l_partkey, sum(l_partkey * (1 + l_suppkey)) AS aggregate FROM lineitem
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GROUP BY l_partkey
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ORDER BY aggregate DESC LIMIT 10;
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-- Enable limit optimization to fetch one third of each shard's data
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SET citus.limit_clause_row_fetch_count TO 600;
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SELECT l_partkey, sum(l_partkey * (1 + l_suppkey)) AS aggregate FROM lineitem
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GROUP BY l_partkey
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ORDER BY aggregate DESC LIMIT 10;
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-- Disable limit optimization for our second test. This time, we have a query
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-- that joins several tables, and that groups and orders the results.
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RESET citus.limit_clause_row_fetch_count;
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SELECT c_custkey, c_name, count(*) as lineitem_count
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FROM customer, orders, lineitem
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WHERE c_custkey = o_custkey AND l_orderkey = o_orderkey
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GROUP BY c_custkey, c_name
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ORDER BY lineitem_count DESC, c_custkey LIMIT 10;
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-- Now, enable limit optimization to fetch half of each task's results.
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SET citus.limit_clause_row_fetch_count TO 150;
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SELECT c_custkey, c_name, count(*) as lineitem_count
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FROM customer, orders, lineitem
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WHERE c_custkey = o_custkey AND l_orderkey = o_orderkey
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GROUP BY c_custkey, c_name
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ORDER BY lineitem_count DESC, c_custkey LIMIT 10;
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-- We now test scenarios where applying the limit optimization wouldn't produce
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-- meaningful results. First, we check that we don't push down the limit clause
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-- for non-commutative aggregates.
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SELECT l_partkey, avg(l_suppkey) FROM lineitem
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GROUP BY l_partkey
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ORDER BY 2 DESC, l_partkey LIMIT 10;
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SELECT l_partkey, stddev(l_suppkey::float8) FROM lineitem
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GROUP BY l_partkey
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ORDER BY 2 DESC NULLS LAST, l_partkey LIMIT 10;
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-- also test that we handle execution on coordinator properly
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SELECT l_partkey, avg(distinct l_suppkey) FROM lineitem
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GROUP BY l_partkey
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ORDER BY 2 DESC, l_partkey LIMIT 10;
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-- Next, check that we don't apply the limit optimization for expressions that
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-- have aggregates within them
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SELECT l_partkey, round(sum(l_suppkey)) AS complex_expression FROM lineitem
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GROUP BY l_partkey
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ORDER BY complex_expression DESC LIMIT 10;
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-- Check that query execution works as expected for other queries without limits
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SELECT count(*) count_quantity, l_quantity FROM lineitem WHERE l_quantity < 10.0
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GROUP BY l_quantity
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ORDER BY count_quantity ASC, l_quantity ASC;
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RESET citus.limit_clause_row_fetch_count;
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RESET client_min_messages;
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