Bug#25530204: THE RESULT OF SUM ON JSON_EXTRACT LOST THE DECIMAL PART

When using the SUM aggregate function on an expression that returns a
JSON value, the result does not have a fractional part.

The reason is that the Item_json_func class does not change
Item::decimals from its initial zero value. This is used to determine
how the result is presented, and when it is zero, the fractional part
is lost.

Fix: Set Item::decimals to NOT_FIXED_DEC (31) in
Item_json_func::resolve_type(). This is the same value that is used
for other string types.

For consistency, setting this member, and other members common to
expressions that return JSON values, is moved into a separate helper
function, Item::set_data_type_json(). Item_json and Item_sum_json are
also updated to using the helper function.

Item_sum_json redundantly set the data type to JSON both in its
constructors and in fix_fields(). The patch removes the code from
fix_fields() so that it is only set once. Similarly, make
Item_json_func set the data type in its constructors instead of in
resolve_type().

Changed test result:

Two test cases in json.json_agg get slightly different output because
the type of 1.0 * JSON_ARRAYAGG(a) changed from DOUBLE(18, 1) to
DOUBLE, and the type of SEC_TO_TIME(JSON_ARRAYAGG(a)) changed from
TIME(6) to TIME.

x.find_doc_proj shows metadata for some results where
"fractional_parts" is changed from 0 to 31 as expected.

Change-Id: I37a24afa463201059b7232583d60a5d2e6774fe0

Author: kahatlen

mysql-test/suite/json/r/json_agg.result

@@ -1112,8 +1112,8 @@ CREATE TABLE t2(gid int, a int);
 INSERT INTO t2(gid, a) VALUES (1, 1), (1, 2), (2, 4), (2, 8);
 SELECT gid, 1.0 * JSON_ARRAYAGG(a) FROM t2 GROUP BY gid;
 gid	1.0 * JSON_ARRAYAGG(a)
-1	0.0
-2	0.0
+1	0
+2	0
 Warnings:
 Warning	3156	Invalid JSON value for CAST to DOUBLE from column json_arrayagg( at row 3
 Warning	3156	Invalid JSON value for CAST to DOUBLE from column json_arrayagg( at row 5
@@ -1142,8 +1142,8 @@ Warning	3156	Invalid JSON value for CAST to DATE/TIME/DATETIME/TIMESTAMP from co
 Warning	3156	Invalid JSON value for CAST to DATE/TIME/DATETIME/TIMESTAMP from column json_arrayagg( at row 5
 SELECT gid, SEC_TO_TIME(JSON_ARRAYAGG(a)) FROM t2 GROUP BY gid;
 gid	SEC_TO_TIME(JSON_ARRAYAGG(a))
-1	00:00:00
-2	00:00:00
+1	00:00:00.000000
+2	00:00:00.000000
 Warnings:
 Warning	3156	Invalid JSON value for CAST to DECIMAL from column json_arrayagg( at row 3
 Warning	3156	Invalid JSON value for CAST to DECIMAL from column json_arrayagg( at row 5

mysql-test/suite/json/r/json_no_table.result

@@ -3332,3 +3332,9 @@ JSON_CONTAINS_PATH('{"a":"x"}', CAST('all' AS CHAR CHARSET utf16),
 '$.a', '$.b') AS c4;
 c1	c2	c3	c4
 "$[0]"	["$[0]", "$[1]"]	1	0
+#
+# Bug#25530204: THE RESULT OF SUM ON JSON_EXTRACT LOST THE DECIMAL PART
+#
+SELECT SUM(CAST('5.45' AS JSON));
+SUM(CAST('5.45' AS JSON))
+5.45

mysql-test/suite/json/t/json_no_table.test

@@ -2329,3 +2329,8 @@ SELECT JSON_SEARCH('["a", "a"]', CAST('one' AS CHAR CHARSET utf16), 'a') AS c1,
                           '$.a', '$.b') AS c3,
        JSON_CONTAINS_PATH('{"a":"x"}', CAST('all' AS CHAR CHARSET utf16),
                           '$.a', '$.b') AS c4;
+
+--echo #
+--echo # Bug#25530204: THE RESULT OF SUM ON JSON_EXTRACT LOST THE DECIMAL PART
+--echo #
+SELECT SUM(CAST('5.45' AS JSON));

rapid/plugin/x/tests/mtr/r/find_doc_proj.result

@@ -152,7 +152,7 @@ Mysqlx.Resultset.ColumnMetaData {
   schema: ""
   catalog: "def"
   collation: 46
-  fractional_digits: 0
+  fractional_digits: 31
   length: 16777216
   flags: 0
   content_type: 2
@@ -221,7 +221,7 @@ Mysqlx.Resultset.ColumnMetaData {
   schema: ""
   catalog: "def"
   collation: 46
-  fractional_digits: 0
+  fractional_digits: 31
   length: 16777216
   flags: 0
   content_type: 2
@@ -294,7 +294,7 @@ Mysqlx.Resultset.ColumnMetaData {
   schema: ""
   catalog: "def"
   collation: 46
-  fractional_digits: 0
+  fractional_digits: 31
   length: 16777216
   flags: 0
   content_type: 2
@@ -351,7 +351,7 @@ Mysqlx.Resultset.ColumnMetaData {
   schema: ""
   catalog: "def"
   collation: 46
-  fractional_digits: 0
+  fractional_digits: 31
   length: 16777216
   flags: 0
   content_type: 2

sql/item.cc

@@ -7368,13 +7368,11 @@ class Item_json final : public Item_basic_constant
 {
   Json_wrapper m_value;
 public:
-  Item_json(Json_wrapper &&value, const Item_name_string &name,
-            const DTCollation &coll)
+  Item_json(Json_wrapper &&value, const Item_name_string &name)
     : m_value(std::move(value))
   {
-    set_data_type(MYSQL_TYPE_JSON);
+    set_data_type_json();
     item_name= name;
-    collation.set(coll);
   }
   enum Type type() const override { return STRING_ITEM; }
 
@@ -7426,7 +7424,7 @@ class Item_json final : public Item_basic_constant
   Item *clone_item() const override
   {
     Json_wrapper wr(m_value.clone_dom(current_thd));
-    return new Item_json(std::move(wr), item_name, collation);
+    return new Item_json(std::move(wr), item_name);
   }
   /* purecov: end */
 };
@@ -9271,8 +9269,7 @@ bool resolve_const_item(THD *thd, Item **ref, Item *comp_item)
       if (item->null_value)
         new_item= new Item_null(item->item_name);
       else
-        new_item= new Item_json(std::move(wr), item->item_name,
-                                item->collation);
+        new_item= new Item_json(std::move(wr), item->item_name);
       break;
     }
     char buff[MAX_FIELD_WIDTH];

sql/item.h

@@ -1186,6 +1186,17 @@ class Item : public Parse_tree_node
     max_length= MAX_DATE_WIDTH + fsp + (fsp > 0 ? 1 : 0);
   }
 
+  /**
+    Set the data type of the Item to be JSON.
+  */
+  void set_data_type_json()
+  {
+    set_data_type(MYSQL_TYPE_JSON);
+    collation.set(&my_charset_utf8mb4_bin, DERIVATION_IMPLICIT);
+    decimals= NOT_FIXED_DEC;
+    max_length= MAX_BLOB_WIDTH;
+  }
+
   /**
     Set type information of Item from "result" information.
     For String types, type is set based on maximum string size.

sql/item_json_func.h

@@ -18,6 +18,7 @@
 
 #include <stddef.h>
 #include <sys/types.h>
+#include <utility>              // std::forward
 
 #include "binary_log_types.h"
 #include "enum_query_type.h"
@@ -142,25 +143,21 @@ class Item_json_func : public Item_func
     override;
 
 public:
-  Item_json_func(THD *thd, const POS &pos, Item *a) : Item_func(pos, a),
-    m_path_cache(thd, 1)
-  {}
-  Item_json_func(THD *thd, const POS &pos, Item *a, Item *b) : Item_func(pos, a, b),
-    m_path_cache(thd, 2)
-  {}
-  Item_json_func(THD *thd, const POS &pos, Item *a, Item *b, Item *c)
-    : Item_func(pos, a, b, c), m_path_cache(thd, 3)
-  {}
-  Item_json_func(THD *thd, const POS &pos, PT_item_list *a) : Item_func(pos, a),
-    m_path_cache(thd, arg_count)
-  {}
+  /**
+    Construct an Item_json_func instance.
+    @param thd   THD handle
+    @param args  arguments to forward to Item_func's constructor
+  */
+  template <typename... Args>
+  Item_json_func(THD *thd, Args&&... args)
+    : Item_func(std::forward<Args>(args)...), m_path_cache(thd, arg_count)
+  {
+    set_data_type_json();
+  }
 
   bool resolve_type(THD *) override
   {
-    set_data_type(MYSQL_TYPE_JSON);
-    max_length= MAX_BLOB_WIDTH;
     maybe_null= true;
-    collation.set(&my_charset_utf8mb4_bin, DERIVATION_IMPLICIT);
     return false;
   }
   enum Item_result result_type() const override { return STRING_RESULT; }

sql/item_sum.cc

@@ -4230,12 +4230,9 @@ bool Item_sum_json::fix_fields(THD *thd, Item **ref)
   if (resolve_type(thd))
     return true;
 
-  set_data_type(MYSQL_TYPE_JSON);
-
   if (check_sum_func(thd, ref))
     return true;
 
-  max_length= MAX_BLOB_WIDTH;
   maybe_null= true;
   null_value= true;
   fixed= true;

sql/item_sum.h

@@ -23,6 +23,7 @@
 #include <math.h>
 #include <stddef.h>
 #include <sys/types.h>
+#include <utility>          // std::forward
 
 #include "binary_log_types.h"
 #include "enum_query_type.h"
@@ -1026,22 +1027,14 @@ class Item_sum_json : public Item_sum
   Json_wrapper m_wrapper;
 
 public:
-  Item_sum_json(THD *thd, Item_sum *item)
-    : Item_sum(thd, item)
-  {
-    set_data_type(MYSQL_TYPE_JSON);
-  }
-
-  Item_sum_json(const POS &pos, Item *a)
-    : Item_sum(pos, a)
-  {
-    set_data_type(MYSQL_TYPE_JSON);
-  }
-
-  Item_sum_json(const POS &pos, Item *a, Item *b)
-    : Item_sum(pos, a, b)
+  /**
+    Construct an Item_sum_json instance.
+    @param args  arguments to forward to Item_sum's constructor
+  */
+  template <typename... Args>
+  Item_sum_json(Args&&... args) : Item_sum(std::forward<Args>(args)...)
   {
-    set_data_type(MYSQL_TYPE_JSON);
+    set_data_type_json();
   }
 
   bool fix_fields(THD *thd, Item **pItem) override;