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Subject [GitHub] [spark] cloud-fan opened a new pull request #26214: [SPARK-29558][SQL] ResolveTables and ResolveRelations should be order-insensitive
Date Tue, 22 Oct 2019 16:33:22 GMT
cloud-fan opened a new pull request #26214: [SPARK-29558][SQL] ResolveTables and ResolveRelations
should be order-insensitive
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   Make `ResolveRelations` the only rule to resolve `UnresolvedRelation`.
   ### Why are the changes needed?
   Please clarify why the changes are needed. For instance,
     1. If you propose a new API, clarify the use case for a new API.
     2. If you fix a bug, you can clarify why it is a bug.
   To resolve an `UnresolvedRelation`, the process is:
   1. try to resolve to (global) temp view first. If it's not a temp view, move on
   2. if the table name specifies a catalog, lookup the table from the specified catalog.
Otherwise, lookup table from the current catalog.
   3. when looking up table from session catalog, return a v1 relation if the table provider
is v1.
   Currently, this process is done by 2 rules: `ResolveTables` and `ResolveRelations`. To
avoid rule conflicts, we add a lot of checks:
   1. `ResolveTables` only resolves `UnresolvedRelation` if it's not a temp view and the resolved
table is not v1.
   2. `ResolveRelations` only resolves `UnresolvedRelation` if the table name has less than
2 parts.
   This requires to run `ResolveTables` before `ResolveRelations`, otherwise we may resolve
a v2 table to a v1 relation.
   One solution is to add more checks in `ResolveRelations` so that it won't return v1 relation
for v2 tables, but the problems are:
   1. `ResolveTables` and `ResolveRelations` become highly coupled. We need to be very careful
to make sure these 2 rules are mutually exclusive.
   2. It's hard for other developers to figure out the completed logic of resolving `UnresolvedRelation`
by reading the code.
   This PR proposes to resolve `UnresolvedRelation` in one place: `ResolveRelations`
   ### Does this PR introduce any user-facing change?
   If yes, please clarify the previous behavior and the change this PR proposes - provide
the console output, description and/or an example to show the behavior difference if possible.
   If no, write 'No'.
   ### How was this patch tested?
   If tests were added, say they were added here. Please make sure to add some test cases
that check the changes thoroughly including negative and positive cases if possible.
   If it was tested in a way different from regular unit tests, please clarify how you tested
step by step, ideally copy and paste-able, so that other reviewers can test and check, and
descendants can verify in the future.
   If tests were not added, please describe why they were not added and/or why it was difficult
to add.
   existing tests

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