Add SQL-compliant handling for json_merge() when arguments are null (#17983)

* Add SQL-compliant handling of json_merge() function with null expressions.

The json_merge() function fail with a query out with JSON_MERGE(null, null):
Function[json_merge] invalid input expected STRING but got STRING instead

The behavior in both mysql and postgres is that the function returns NULL
if any argument is NULL. This change adds that to align and make it SQL-compliant.

* One more doc change.

Author: abhishekrb19

docs/querying/sql-functions.md

@@ -3235,6 +3235,7 @@ Returns the following:
 ## JSON_MERGE
 
 Merges two or more JSON `STRING` or `COMPLEX<json>` expressions into one, preserving the rightmost value when there are key overlaps.
+Returns `NULL` if any argument is `NULL`.
 The function always returns a `COMPLEX<json>` object.
 
 * **Syntax:** `JSON_MERGE(expr1, expr2[, expr3 ...])`

docs/querying/sql-json-functions.md

@@ -34,10 +34,11 @@ Druid supports nested columns, which provide optimized storage and indexes for n
 
 You can use the following JSON functions to extract, transform, and create `COMPLEX<json>` objects.
 
+
 | Function | Notes |
 | --- | --- |
 |`JSON_KEYS(expr, path)`| Returns an array of field names from `expr` at the specified `path`.|
-|`JSON_MERGE(expr1, expr2[, expr3 ...])`| Merges two or more JSON `STRING` or `COMPLEX<json>` values into one, preserving the rightmost value when there are key overlaps. Always returns a `COMPLEX<json>` object.|
+|`JSON_MERGE(expr1, expr2[, expr3 ...])`| Merges two or more JSON `STRING` or `COMPLEX<json>` values into one, preserving the rightmost value when there are key overlaps. Returns `NULL` if any argument is `NULL`. Always returns a `COMPLEX<json>` object.|
 |`JSON_OBJECT(KEY expr1 VALUE expr2[, KEY expr3 VALUE expr4, ...])` | Constructs a new `COMPLEX<json>` object from one or more expressions. The `KEY` expressions must evaluate to string types. The `VALUE` expressions can be composed of any input type, including other `COMPLEX<json>` objects. The function can accept colon-separated key-value pairs. The following syntax is equivalent: `JSON_OBJECT(expr1:expr2[, expr3:expr4, ...])`.|
 |`JSON_PATHS(expr)`| Returns an array of all paths which refer to literal values in `expr` in JSONPath format. |
 |`JSON_QUERY(expr, path)`| Extracts a `COMPLEX<json>` value from `expr`, at the specified `path`. |
@@ -71,4 +72,4 @@ Consider the following example input JSON:
 - For a key that contains an array, to return the entire array:<br />
   `$['y']` -> `[1, 2, 3]`
 - For a key that contains an array, to return an item in the array:<br />
-  `$.y[1]` -> `2`
+  `$.y[1]` -> `2`

processing/src/main/java/org/apache/druid/query/expression/NestedDataExpressions.java

@@ -138,18 +138,13 @@ public ParseJsonExpr(List<Expr> args)
         public ExprEval eval(ObjectBinding bindings)
         {
           ExprEval arg = args.get(0).eval(bindings);
-          Object obj;
-
-          if (arg.value() == null) {
-            throw JsonMergeExprMacro.this.validationFailed(
-              "invalid input expected %s but got %s instead",
-                ExpressionType.STRING,
-                arg.type()
-            );
+          String argAsJson = getArgAsJson(arg);
+          if (argAsJson == null) {
+            return ExprEval.ofComplex(ExpressionType.NESTED_DATA, null);
           }
-
+          Object obj;
           try {
-            obj = jsonMapper.readValue(getArgAsJson(arg), Object.class);
+            obj = jsonMapper.readValue(argAsJson, Object.class);
           }
           catch (JsonProcessingException e) {
             throw JsonMergeExprMacro.this.processingFailed(e, "bad string input [%s]", arg.asString());
@@ -159,12 +154,13 @@ public ExprEval eval(ObjectBinding bindings)
 
           for (int i = 1; i < args.size(); i++) {
             ExprEval argSub = args.get(i).eval(bindings);
-            
+            String str = getArgAsJson(argSub);
+            if (str == null) {
+              return ExprEval.ofComplex(ExpressionType.NESTED_DATA, null);
+            }
             try {
-              String str = getArgAsJson(argSub);
-              if (str != null) {
-                obj = updater.readValue(str);
-              }
+              obj = updater.readValue(str);
+              updater = jsonMapper.readerForUpdating(obj);
             }
             catch (JsonProcessingException e) {
               throw JsonMergeExprMacro.this.processingFailed(e, "bad string input [%s]", argSub.asString());
@@ -181,6 +177,7 @@ public ExpressionType getOutputType(InputBindingInspector inspector)
           return ExpressionType.NESTED_DATA;
         }
 
+        @Nullable
         private String getArgAsJson(ExprEval arg)
         {
           if (arg.value() == null) {

processing/src/test/java/org/apache/druid/query/expression/NestedDataExpressionsTest.java

@@ -123,7 +123,7 @@ public void testJsonMergeExpression() throws JsonProcessingException
 
     expr = Parser.parse("json_merge('{\"a\":\"x\"}', null)", MACRO_TABLE);
     eval = expr.eval(inputBindings);
-    Assert.assertEquals("{\"a\":\"x\"}", JSON_MAPPER.writeValueAsString(eval.value()));
+    Assert.assertEquals("null", JSON_MAPPER.writeValueAsString(eval.value()));
     Assert.assertEquals(ExpressionType.NESTED_DATA, eval.type());
 
     expr = Parser.parse("json_merge('{\"a\":\"x\"}','{\"b\":\"y\"}','{\"c\":[1,2,3]}')", MACRO_TABLE);
@@ -147,6 +147,55 @@ public void testJsonMergeExpression() throws JsonProcessingException
     Assert.assertEquals(ExpressionType.NESTED_DATA, eval.type());
   }
 
+  @Test
+  public void testJsonMergeWithNullAndEmptyExpressions() throws JsonProcessingException
+  {
+    Expr expr = Parser.parse("json_merge(null, null)", MACRO_TABLE);
+    ExprEval eval = expr.eval(inputBindings);
+    Assert.assertEquals("null", JSON_MAPPER.writeValueAsString(eval.value()));
+    Assert.assertEquals(ExpressionType.NESTED_DATA, eval.type());
+
+    expr = Parser.parse("json_merge(null, '{\"a\":\"x\"}')", MACRO_TABLE);
+    eval = expr.eval(inputBindings);
+    Assert.assertEquals("null", JSON_MAPPER.writeValueAsString(eval.value()));
+    Assert.assertEquals(ExpressionType.NESTED_DATA, eval.type());
+
+    expr = Parser.parse("json_merge('{\"a\":\"x\"}', null)", MACRO_TABLE);
+    eval = expr.eval(inputBindings);
+    Assert.assertEquals("null", JSON_MAPPER.writeValueAsString(eval.value()));
+    Assert.assertEquals(ExpressionType.NESTED_DATA, eval.type());
+
+    expr = Parser.parse("json_merge('{\"a\":\"x\"}', null, null, null)", MACRO_TABLE);
+    eval = expr.eval(inputBindings);
+    Assert.assertEquals("null", JSON_MAPPER.writeValueAsString(eval.value()));
+    Assert.assertEquals(ExpressionType.NESTED_DATA, eval.type());
+
+    expr = Parser.parse("json_merge('{\"a\":\"x\"}', null, null, json_object())", MACRO_TABLE);
+    eval = expr.eval(inputBindings);
+    Assert.assertEquals("null", JSON_MAPPER.writeValueAsString(eval.value()));
+    Assert.assertEquals(ExpressionType.NESTED_DATA, eval.type());
+
+    expr = Parser.parse("json_merge(json_object(), json_object(), json_object())", MACRO_TABLE);
+    eval = expr.eval(inputBindings);
+    Assert.assertEquals("{}", JSON_MAPPER.writeValueAsString(eval.value()));
+    Assert.assertEquals(ExpressionType.NESTED_DATA, eval.type());
+
+    expr = Parser.parse("json_merge(json_object(), json_object(), json_object(), null)", MACRO_TABLE);
+    eval = expr.eval(inputBindings);
+    Assert.assertEquals("null", JSON_MAPPER.writeValueAsString(eval.value()));
+    Assert.assertEquals(ExpressionType.NESTED_DATA, eval.type());
+
+    expr = Parser.parse("json_merge(json_object(), json_object(), json_object(), coalesce(null, '{}'))", MACRO_TABLE);
+    eval = expr.eval(inputBindings);
+    Assert.assertEquals("{}", JSON_MAPPER.writeValueAsString(eval.value()));
+    Assert.assertEquals(ExpressionType.NESTED_DATA, eval.type());
+
+    expr = Parser.parse("json_merge(coalesce(null, '{}'), '{\"a\":\"x\"}')", MACRO_TABLE);
+    eval = expr.eval(inputBindings);
+    Assert.assertEquals("{\"a\":\"x\"}", JSON_MAPPER.writeValueAsString(eval.value()));
+    Assert.assertEquals(ExpressionType.NESTED_DATA, eval.type());
+  }
+
   @Test
   public void testJsonMergeOverflow() throws JsonProcessingException
   {