MDEV-6109: EXPLAIN JSON

- Add first testcases
- Don't overquote when printing conditions
- Other small output fixes

mysql-test/r/explain_json.result

+drop table if exists t0,t1;
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+explain format=json select * from t0;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t0",
+      "access_type": "ALL",
+      "rows": 10,
+      "filtered": 100
+    }
+  }
+}
+explain format=json select * from t0 where 1>2;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "message": "Impossible WHERE"
+    }
+  }
+}
+explain format=json select * from t0 where a<3;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t0",
+      "access_type": "ALL",
+      "rows": 10,
+      "filtered": 100,
+      "attached_condition": "(t0.a < 3)"
+    }
+  }
+}
+drop table t0;

mysql-test/t/explain_json.test

+#
+# EXPLAIN FORMAT=JSON tests. These are tests developed for MariaDB.
+#
+--disable_warnings
+drop table if exists t0,t1;
+--enable_warnings
+
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+explain format=json select * from t0;
+
+explain format=json select * from t0 where 1>2;
+
+explain format=json select * from t0 where a<3;
+
+#create table t1 (a int, b int, filler char(32), key(a));
+#insert into t1 
+#select 
+#  A.a + B.a* 10 + C.a * 100, 
+#  A.a + B.a* 10 + C.a * 100,
+#  'filler'
+#from t0 A, t0 B, t0 C;
+#
+#explain format=json select * from t0,t1 where t1.a=t0.a;
+
+drop table t0;

sql/item.cc

@@ -2583,7 +2583,13 @@ void Item_ident::print(String *str, enum_query_type query_type)
   }
   if (db_name && db_name[0] && !alias_name_used)
   {
-    if (!(cached_table && cached_table->belong_to_view &&
+    /* 
+      When printing EXPLAIN, don't print database name when it's the same as
+      current database.
+    */
+    bool skip_db= (query_type & QT_EXPLAIN) && !strcmp(thd->db, db_name);
+    if (!skip_db && 
+        !(cached_table && cached_table->belong_to_view &&
           cached_table->belong_to_view->compact_view_format))
     {
       append_identifier(thd, str, d_name, (uint)strlen(d_name));

sql/my_json_writer.cc

@@ -100,7 +100,7 @@ void Json_writer::add_double(double val)
     start_element();
 
   char buf[64];
-  my_snprintf(buf, sizeof(buf), "%lf", val);
+  my_snprintf(buf, sizeof(buf), "%lg", val);
   output.append(buf);
   element_started= false;
 }

sql/my_json_writer.h

 /* Todo: SkySQL copyrights */
 
+
+/*
+  A class to write well-formed JSON documents. The documents are also formatted
+  for human readability.
+*/
+
 class Json_writer
 {
 public:
@@ -25,7 +31,7 @@ class Json_writer
     first_child(true)
   {}
 private:
-  // stack of (name, bool is_object_or_array) elements.
+  // TODO: a stack of (name, bool is_object_or_array) elements.
   int indent_level;
   enum { INDENT_SIZE = 2 };
   

sql/mysqld.h

@@ -611,9 +611,13 @@ enum enum_query_type
   /// Without character set introducers.
   QT_WITHOUT_INTRODUCERS= (1 << 1),
   /// view internal representation (like QT_ORDINARY except ORDER BY clause)
-  QT_VIEW_INTERNAL= (1 << 2)
+  QT_VIEW_INTERNAL= (1 << 2),
+  /// This value means focus on readability, not on ability to parse back, etc.
+  QT_EXPLAIN= (1 << 4)
 };
 
+
+
 /* query_id */
 typedef int64 query_id_t;
 extern query_id_t global_query_id;

sql/sql_explain.cc

@@ -774,10 +774,17 @@ int Explain_table_access::print_explain(select_result_sink *output, uint8 explai
 
 static void write_item(Json_writer *writer, Item *item)
 {
+  THD *thd= current_thd;
   char item_buf[256];
   String str(item_buf, sizeof(item_buf), &my_charset_bin);
   str.length(0);
-  item->print(&str ,QT_ORDINARY);
+
+  ulonglong save_option_bits= thd->variables.option_bits;
+  thd->variables.option_bits &= ~OPTION_QUOTE_SHOW_CREATE;
+
+  item->print(&str, QT_EXPLAIN);
+
+  thd->variables.option_bits= save_option_bits;
   writer->add_str(str.c_ptr_safe());
 }
 
@@ -848,7 +855,8 @@ void Explain_table_access::print_explain_json(Json_writer *writer,
     writer->add_member("key").add_str(key_str);
 
   /* `used_key_parts` */
-  writer->add_member("used_key_parts").add_str("TODO");
+  if (key_str.length())
+    writer->add_member("used_key_parts").add_str("TODO");
 
   StringBuffer<64> key_len_str;
   fill_key_len_str(&key_len_str);

sql/sql_parse.cc

@@ -5269,21 +5269,9 @@ static bool execute_sqlcom_select(THD *thd, TABLE_LIST *all_tables)
           top-level LIMIT
         */        
         result->reset_offset_limit(); 
-        if (thd->lex->explain_json)
+        if (lex->explain_json)
         {
-          /*
-          Json_writer writer;
-          writer.start_object();
-          thd->lex->explain->print_explain_json(&writer, thd->lex->analyze_stmt);
-          writer.end_object();
-
-          const CHARSET_INFO *cs= system_charset_info;
-          List<Item> item_list;
-          String *buf= &writer.output;
-          item_list.push_back(new Item_string(buf->ptr(), buf->length(), cs));
-          result->send_data(item_list);
-          */
-          thd->lex->explain->print_explain_json(result, thd->lex->analyze_stmt);
+          lex->explain->print_explain_json(result, lex->analyze_stmt);
         }
         else
         {