Add tests for CASE decomposition. (#15639)

I was looking into adding a rule to do this, and found that it was already
happening as part of Calcite's RexSimplify. So this patch simply adds some
tests to ensure that it continues to happen.

sql/src/test/java/org/apache/druid/sql/calcite/CalciteQueryTest.java

@@ -3209,6 +3209,88 @@ public class CalciteQueryTest extends BaseCalciteQueryTest
     );
   }
 
+  @Test
+  public void testDecomposeCaseWhenThreeArg()
+  {
+    // Cannot vectorize due to virtual columns.
+    cannotVectorize();
+
+    testQuery(
+        "SELECT\n"
+        + "  dim1, dim2, CASE WHEN dim1 = 'abc' THEN dim1 ELSE dim2 END\n"
+        + "FROM druid.foo\n"
+        + "WHERE\n"
+        + "  CASE WHEN dim1 = 'abc' THEN dim1 ELSE dim2 END = 'abc'",
+        ImmutableList.of(
+            newScanQueryBuilder()
+                .dataSource(CalciteTests.DATASOURCE1)
+                .virtualColumns(
+                    expressionVirtualColumn(
+                        "v0",
+                        "case_searched((\"dim1\" == 'abc'),\"dim1\",\"dim2\")",
+                        ColumnType.STRING
+                    )
+                )
+                .intervals(querySegmentSpec(Filtration.eternity()))
+                .filters(
+                    or(
+                        equality("dim1", "abc", ColumnType.STRING),
+                        and(
+                            equality("dim2", "abc", ColumnType.STRING),
+                            NullHandling.sqlCompatible()
+                            ? not(istrue(equality("dim1", "abc", ColumnType.STRING)))
+                            : not(equality("dim1", "abc", ColumnType.STRING))
+                        )
+                    )
+                )
+                .columns("dim1", "dim2", "v0")
+                .build()
+        ),
+        ImmutableList.of(
+            new Object[]{"def", "abc", "abc"},
+            new Object[]{"abc", NullHandling.defaultStringValue(), "abc"}
+        )
+    );
+  }
+
+  @Test
+  public void testDecomposeCaseWhenTwoArg()
+  {
+    // Cannot vectorize due to virtual columns.
+    cannotVectorize();
+
+    testQuery(
+        "SELECT\n"
+        + "  dim1, dim2, CASE WHEN dim1 = 'def' THEN dim2 END\n"
+        + "FROM druid.foo\n"
+        + "WHERE\n"
+        + "  CASE WHEN dim1 = 'def' THEN dim2 END = 'abc'",
+        ImmutableList.of(
+            newScanQueryBuilder()
+                .dataSource(CalciteTests.DATASOURCE1)
+                .virtualColumns(
+                    expressionVirtualColumn(
+                        "v0",
+                        "case_searched((\"dim1\" == 'def'),\"dim2\",null)",
+                        ColumnType.STRING
+                    )
+                )
+                .intervals(querySegmentSpec(Filtration.eternity()))
+                .filters(
+                    and(
+                        equality("dim1", "def", ColumnType.STRING),
+                        equality("dim2", "abc", ColumnType.STRING)
+                    )
+                )
+                .columns("dim1", "dim2", "v0")
+                .build()
+        ),
+        ImmutableList.of(
+            new Object[]{"def", "abc", "abc"}
+        )
+    );
+  }
+
   @Test
   public void testGroupByCaseWhenOfTripleAnd()
   {