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(Based on a patch by Yuchen Pei) When computing table dependencies in simplify_joins(), do not make LEFT JOIN-ed table (or join nest) to be dependent on all preceding tables. For queries in form t1 LEFT JOIN t2 ON t2.col=t1.col LEFT JOIN t3 ON t3.col=t1.col this allows the optimizer to construct join order of t1-t3-t2. Before this patch, t1-t2-t3 was the only possible order. Note that queries that use Oracle's Outer Join syntax (col1(+)=col2) are converted into chained independent LEFT JOINs like the above. The optimization is controlled by optimizer_switch='reorder_outer_joins=ON' It is NOT enabled by default as it is known to expose problems with join pruning. It is advised to set optimizer_prune_level=0 when setting reorder_outer_joins=on. Co-authored-by: Yuchen Pei <ycp@mariadb.com> |
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