MDEV-37933: Rewrite [NATURAL] FULL OUTER to LEFT, RIGHT, or INNER JOIN #4448
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DaveGosselin-MariaDB
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spetrunia
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| // Only the right table in a LEFT JOIN has the naming context in the grammar | ||
| left_table->on_context= nullptr; | ||
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| // The grammar 'search_condition: ' rule marks this. |
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So, an Item* object has been a FULL OUTER JOIN's ON expression and didn't have IS_COND flag?
But now it becomes a LEFT JOIN's ON expression and we're setting the flag.
A Naive question is, why don't we set IS_COND flag for FULL OUTER JOINs' ON expressions?
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We could do that but it would be nice if the grammar was not responsible for so much. I think it's better to keep the FULL JOIN rewrite bits all together in their respective functions. For example I would not like the grammar to call convert_right_join for RIGHT JOINs but defer that until after lexing is completed.
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Rewrite FULL OUTER JOIN queries as either LEFT, RIGHT, or INNER JOIN by checking if and how the WHERE clause rejects nulls. For example, the following two queries are equivalent because the WHERE condition rejects nulls from the left table and allows matches in the right table (or NULL from the right table) for the remaining rows: SELECT * FROM t1 FULL JOIN t2 ON t1.v = t2.v WHERE t1.v IS NOT NULL; SELECT * FROM t1 LEFT JOIN t2 ON t1.v = t2.v;
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Rewrite FULL OUTER JOIN queries as either LEFT, RIGHT, or INNER JOIN by checking if and how the WHERE clause rejects nulls.
For example, the following two queries are equivalent because the WHERE condition rejects nulls from the left table and allows matches in the right table (or NULL from the right table) for the remaining rows: