[GitHub] [spark] maropu commented on a change in pull request #27935: [SPARK-25121][SQL] Supports multi-part table names for broadcast hint resolution

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maropu commented on a change in pull request #27935: [SPARK-25121][SQL] Supports multi-part table names for broadcast hint resolution
URL: https://github.com/apache/spark/pull/27935#discussion_r394103417
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/ResolveHints.scala
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 @@ -64,31 +64,63 @@ object ResolveHints {
             _.toUpperCase(Locale.ROOT)).contains(hintName.toUpperCase(Locale.ROOT))))
     }
 
+    // This method checks if given multi-part identifiers are matched with each other.
+    // The [[ResolveJoinStrategyHints]] rule is applied before the resolution batch
+    // in the analyzer and we cannot semantically compare them at this stage.
+    // Therefore, we follow simple rules; the process is independent of a session catalog
+    // (`currentDb` in [[SessionCatalog]]) and it just compares them literally.
+    //
+    // Specifically, we follow simple three rules below:
+    //
+    //  1. they match if an identifier in a hint only has one part and it is the same with
+    //     a relation name in a query. If a relation has a namespace (`db1.t`), we just ignore it.
+    //     For example, in a query `SELECT /* BROADCAST(t) */ * FROM db1.t JOIN t`,
+    //     the broadcast hint will match both tables, `db1.t` and `t`.
+    //
+    //  2. they match if an identifier in a hint has two parts and it is the same with
+    //     a two part identifier (name and namespace) in a relation.
+    //     For example, in a query `SELECT /* BROADCAST(default.t) */ * FROM default.t JOIN t`,
+    //     the broadcast hint will match the left-side table only, `default.t`.
+    //
+    //  3. otherwise, no match happens.
 
 Review comment:
   I re-read your comments again and summarized up them above. If I misunderstand something, please let me know. @dongjoon-hyun @cloud-fan 

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