Handle reused symbols in ProjectNode #4667
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Fixes processing of ProjectNode with reused symbols In UnaliasSymbolReferences: Assignment of a form `s -> x` establishes new semantics for symbol s. It is possible though that symbol `s` is present in the source plan, and represents the same or different semantics. As a consequence, any symbol mapping derived from the source plan involving symbol `s` becomes potentially invalid, e.g. `s -> y` or `y -> s` refer to the old semantics of symbol `s`. In such case, the underlying mappings are only used to map projection assignments' values to ensure consistency with the source plan. They aren't used to map projection outputs to avoid: - errors from duplicate assignments - incorrect results from mixed semantics of symbols Also, the underlying mappings aren't passed up the plan, and new mappings aren't derived from projection assignments (with the exception for "deduplicating" mappings for repeated assignments). This can be thought of as a "cut-off" at the point of potentially changed semantics. Note: the issue of ambiguous symbols does not apply to symbols involved in context (correlation) mapping. Those symbols are supposed to represent constant semantics throughout the plan.
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Fixes processing of ProjectNode with reused symbols In UnaliasSymbolReferences:
Assignment of a form
s -> xestablishes new semantics for symbol s.It is possible though that symbol
sis present in the source plan,and represents the same or different semantics.
As a consequence, any symbol mapping derived from the source plan
involving symbol
sbecomes potentially invalid,e.g.
s -> yory -> srefer to the old semantics of symbols.In such case, the underlying mappings are only used to map
projection assignments' values to ensure consistency with the source plan.
They aren't used to map projection outputs to avoid:
Also, the underlying mappings aren't passed up the plan, and new mappings
aren't derived from projection assignments (with the exception for "deduplicating"
mappings for repeated assignments).
This can be thought of as a "cut-off" at the point of potentially changed semantics.
Note: the issue of ambiguous symbols does not apply to symbols involved in
context (correlation) mapping. Those symbols are supposed to represent constant
semantics throughout the plan.
change is needed for #4250, and will be tested there.