[CALCITE-7440] Fix rel2sql correlation fallback without JDBC regressions

Author: bvolpato

core/src/main/java/org/apache/calcite/rel/rel2sql/RelToSqlConverter.java

@@ -54,14 +54,11 @@
 import org.apache.calcite.rel.type.RelDataTypeField;
 import org.apache.calcite.rex.RexBuilder;
 import org.apache.calcite.rex.RexCall;
-import org.apache.calcite.rex.RexCorrelVariable;
-import org.apache.calcite.rex.RexFieldAccess;
 import org.apache.calcite.rex.RexInputRef;
 import org.apache.calcite.rex.RexLiteral;
 import org.apache.calcite.rex.RexLocalRef;
 import org.apache.calcite.rex.RexNode;
 import org.apache.calcite.rex.RexProgram;
-import org.apache.calcite.rex.RexSubQuery;
 import org.apache.calcite.sql.JoinConditionType;
 import org.apache.calcite.sql.JoinType;
 import org.apache.calcite.sql.SqlAsofJoin;
@@ -560,7 +557,6 @@ public Result visit(Filter e) {
           visitInput(e, 0, isAnon(), ignoreClauses,
               ImmutableSet.of(Clause.HAVING));
       parseCorrelTable(e, x);
-      parseCorrelVariables(e.getCondition());
       final Builder builder = x.builder(e);
       x.asSelect().setHaving(
           SqlUtil.andExpressions(x.asSelect().getHaving(),
@@ -573,7 +569,6 @@ public Result visit(Filter e) {
       }
       parseCorrelTable(e, x);
       final Builder builder = x.builder(e);
-      parseCorrelVariables(e.getCondition());
       if (input instanceof Join) {
         final Context context = x.qualifiedContext();
         if (selectListRequired(context)) {
@@ -609,31 +604,6 @@ private static boolean selectListRequired(Context context) {
     return false;
   }
 
-  private void parseCorrelVariables(RexNode rexNode) {
-    if (rexNode instanceof RexCorrelVariable) {
-      final RexCorrelVariable correl = (RexCorrelVariable) rexNode;
-      correlTableMap.putIfAbsent(
-          correl.id,
-          aliasContext(ImmutableMap.of(correl.id.getName(), correl.getType()), true));
-      return;
-    }
-    if (rexNode instanceof RexSubQuery) {
-      for (RexNode operand : ((RexSubQuery) rexNode).operands) {
-        parseCorrelVariables(operand);
-      }
-      return;
-    }
-    if (rexNode instanceof RexFieldAccess) {
-      parseCorrelVariables(((RexFieldAccess) rexNode).getReferenceExpr());
-      return;
-    }
-    if (rexNode instanceof RexCall) {
-      for (RexNode operand : ((RexCall) rexNode).operands) {
-        parseCorrelVariables(operand);
-      }
-    }
-  }
-
   /**
    * Extracts the table name from a SqlNode if it represents a simple table reference.
    * Returns null for complex nodes like subqueries or joins.

core/src/main/java/org/apache/calcite/rel/rel2sql/SqlImplementor.java

@@ -1587,9 +1587,42 @@ public static SqlNode toSql(RexLiteral literal) {
   }
 
   protected Context getAliasContext(RexCorrelVariable variable) {
-    return requireNonNull(
-        correlTableMap.get(variable.id),
-        () -> "variable " + variable.id + " is not found");
+    Context context = correlTableMap.get(variable.id);
+    if (context == null) {
+      // Some rewrites can introduce a new correlation id that still points to
+      // the same outer scope. Reuse a compatible known context before failing.
+      context = findCompatibleCorrelContext(variable.id, variable.getType().getFieldCount());
+      if (context == null && correlTableMap.isEmpty()) {
+        context =
+            aliasContext(ImmutableMap.of(variable.id.getName(), variable.getType()), true);
+      }
+      if (context != null) {
+        correlTableMap.put(variable.id, context);
+      }
+    }
+    return requireNonNull(context, () -> "variable " + variable.id + " is not found");
+  }
+
+  private @Nullable Context findCompatibleCorrelContext(CorrelationId missingId,
+      int expectedFieldCount) {
+    Context fallback = null;
+    String fallbackName = null;
+    for (Map.Entry<CorrelationId, Context> entry : correlTableMap.entrySet()) {
+      final CorrelationId id = entry.getKey();
+      final Context context = entry.getValue();
+      if (id.equals(missingId)) {
+        continue;
+      }
+      if (context.fieldCount == expectedFieldCount) {
+        return context;
+      }
+      final String name = id.getName();
+      if (fallbackName == null || name.compareTo(fallbackName) < 0) {
+        fallbackName = name;
+        fallback = context;
+      }
+    }
+    return fallback;
   }
 
   /** Simple implementation of {@link Context} that cannot handle sub-queries