json_typeof function.

Andrew Tipton.

src/backend/utils/adt/json.c

@@ -1826,3 +1826,55 @@ escape_json(StringInfo buf, const char *str)
 	}
 	appendStringInfoCharMacro(buf, '\"');
 }
+
+/*
+ * SQL function json_typeof(json) -> text
+ *
+ * Returns the type of the outermost JSON value as TEXT.  Possible types are
+ * "object", "array", "string", "number", "boolean", and "null".
+ * 
+ * Performs a single call to json_lex() to get the first token of the supplied
+ * value.  This initial token uniquely determines the value's type.  As our
+ * input must already have been validated by json_in() or json_recv(), the
+ * initial token should never be JSON_TOKEN_OBJECT_END, JSON_TOKEN_ARRAY_END,
+ * JSON_TOKEN_COLON, JSON_TOKEN_COMMA, or JSON_TOKEN_END.
+ */
+Datum
+json_typeof(PG_FUNCTION_ARGS)
+{
+    text       *json = PG_GETARG_TEXT_P(0);
+
+    JsonLexContext *lex = makeJsonLexContext(json, false);
+    JsonTokenType tok;
+    char *type;
+
+    /* Lex exactly one token from the input and check its type. */
+    json_lex(lex);
+    tok = lex_peek(lex);
+    switch (tok)
+    {
+        case JSON_TOKEN_OBJECT_START:
+            type = "object";
+            break;
+        case JSON_TOKEN_ARRAY_START:
+            type = "array";
+            break;
+        case JSON_TOKEN_STRING:
+            type = "string";
+            break;
+        case JSON_TOKEN_NUMBER:
+            type = "number";
+            break;
+        case JSON_TOKEN_TRUE:
+        case JSON_TOKEN_FALSE:
+            type = "boolean";
+            break;
+        case JSON_TOKEN_NULL:
+            type = "null";
+            break;
+        default:
+            elog(ERROR, "unexpected json token: %d", tok);
+    }
+
+    PG_RETURN_TEXT_P(cstring_to_text(type));
+}