BUG#25575: ERROR 1052 (Column in from clause is ambiguous) with sub-join
Two problems here: Problem 1: While constructing the join columns list the optimizer does as follows: 1. Sets the join_using_fields/natural_join members of the right JOIN operand. 2. Makes a "table reference" (TABLE_LIST) to parent the two tables. 3. Assigns the join_using_fields/is_natural_join of the wrapper table using join_using_fields/natural_join of the rightmost table 4. Sets join_using_fields to NULL for the right JOIN operand. 5. Passes the parent table up to the same procedure on the upper level. Step 1 overrides the the join_using_fields that are set for a nested join wrapping table in step 4. Fixed by making a designated variable SELECT_LEX::prev_join_using to pass the data from step 1 to step 4 without destroying the wrapping table data. Problem 2: The optimizer checks for ambiguous columns while transforming NATURAL JOIN/JOIN USING to JOIN ON. While doing that there was no distinction between columns that are used in the generated join condition (where ambiguity can be checked) and the other columns (where ambiguity can be checked only when resolving references coming from outside the JOIN construct itself). Fixed by allowing the non-USING columns to be present in multiple copies in both sides of the join and moving the ambiguity check to the place where unqualified references to the join columns are resolved (find_field_in_natural_join()).
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