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Join Operations

How to Force a Join Order

DuckDB has a cost-based query optimizer, which uses statistics in the base tables (stored in a DuckDB database or Parquet files) to estimate the cardinality of operations.

Turn off the Join Order Optimizer

To turn off the join order optimizer, set the following PRAGMAs:

SET disabled_optimizers = 'join_order,build_side_probe_side';

This disables both the join order optimizer and left/right swapping for joins. This way, DuckDB builds a left-deep join tree following the order of JOIN clauses.

SELECT …
FROM …
JOIN … -- this join is performed first
JOIN … -- this join is performed second

Once the query in question has been executed, turn back the optimizers with the following command:

SET disabled_optimizers = '';

Create Temporary Tables

To force a particular join order, you can break up the query into multiple queries with each creating a temporary tables:

CREATE OR REPLACE TEMPORARY TABLE t1 AS
    …;

-- join on the result of the first query, t1
CREATE OR REPLACE TEMPORARY TABLE t2 AS
    SELECT * FROM t1 …;

-- compute the final result using t2
SELECT * FROM t1 …

To clean up, drop the interim tables:

DROP TABLE IF EXISTS t1;
DROP TABLE IF EXISTS t2;
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