MDEV-14275 Improving memory utilization for information schema

SELECT queries on information_schema table will now not store data
in the temporary table for columns that are not used in the query.
This can drastically reduce memory when there are many rows involved
in a query, like using
SELECT table_name FROM information_schema.table
on a system with many tables.
The difference for the above query is about 14K per existing table.

The code works this way:
- Create a bitmap for all information_schema fields used in the query
- For BLOB and VARCHAR fields that are not use, create a Field_null field.

sql/sql_select.cc

@@ -16685,11 +16685,15 @@ Field *Item::create_field_for_schema(THD *thd, TABLE *table)
   {
     Field *field;
     if (max_length > MAX_FIELD_VARCHARLENGTH)
-      field= new Field_blob(max_length, maybe_null, &name,
-                            collation.collation);
+      field= new (thd->mem_root) Field_blob(max_length, maybe_null, &name,
+                                            collation.collation);
+    else if (max_length > 0)
+      field= new (thd->mem_root) Field_varstring(max_length, maybe_null, &name,
+                                                 table->s,
+                                                 collation.collation);
     else
-      field= new Field_varstring(max_length, maybe_null, &name,
-                                 table->s, collation.collation);
+      field= new Field_null((uchar*) 0, 0, Field::NONE, &name,
+                            collation.collation);
     if (field)
       field->init(table);
     return field;

sql/sql_show.cc

@@ -8071,6 +8071,50 @@ ST_SCHEMA_TABLE *get_schema_table(enum enum_schema_tables schema_table_idx)
   return &schema_tables[schema_table_idx];
 }
 
+static void
+mark_all_fields_used_in_query(THD *thd,
+                              ST_FIELD_INFO *schema_fields,
+                              MY_BITMAP *bitmap,
+                              Item *all_items)
+{
+  Item *item;
+  DBUG_ENTER("mark_all_fields_used_in_query");
+
+  /* If not SELECT command, return all columns */
+  if (thd->lex->sql_command != SQLCOM_SELECT &&
+      thd->lex->sql_command != SQLCOM_SET_OPTION)
+  {
+    bitmap_set_all(bitmap);
+    DBUG_VOID_RETURN;
+  }
+
+  for (item= all_items ; item ; item= item->next)
+  {
+    if (item->type() == Item::FIELD_ITEM)
+    {
+      ST_FIELD_INFO *fields= schema_fields;
+      uint count;
+      Item_field *item_field= (Item_field*) item;
+
+      /* item_field can be '*' as this function is called before fix_fields */
+      if (item_field->field_name.str == star_clex_str.str)
+      {
+        bitmap_set_all(bitmap);
+        break;
+      }
+      for (count=0; fields->field_name; fields++, count++)
+      {
+        if (!my_strcasecmp(system_charset_info, fields->field_name,
+                           item_field->field_name.str))
+        {
+          bitmap_set_bit(bitmap, count);
+          break;
+        }
+      }
+    }
+  }
+  DBUG_VOID_RETURN;
+}
 
 /**
   Create information_schema table using schema_table data.
@@ -8095,17 +8139,34 @@ ST_SCHEMA_TABLE *get_schema_table(enum enum_schema_tables schema_table_idx)
 
 TABLE *create_schema_table(THD *thd, TABLE_LIST *table_list)
 {
-  int field_count= 0;
-  Item *item;
+  uint field_count;
+  Item *item, *all_items;
   TABLE *table;
   List<Item> field_list;
   ST_SCHEMA_TABLE *schema_table= table_list->schema_table;
   ST_FIELD_INFO *fields_info= schema_table->fields_info;
+  ST_FIELD_INFO *fields;
   CHARSET_INFO *cs= system_charset_info;
   MEM_ROOT *mem_root= thd->mem_root;
+  MY_BITMAP bitmap;
+  my_bitmap_map *buf;
   DBUG_ENTER("create_schema_table");
 
-  for (; fields_info->field_name; fields_info++)
+  for (field_count= 0, fields= fields_info; fields->field_name; fields++)
+    field_count++;
+  if (!(buf= (my_bitmap_map*) thd->alloc(bitmap_buffer_size(field_count))))
+    DBUG_RETURN(NULL);
+  my_bitmap_init(&bitmap, buf, field_count, 0);
+
+  if (!thd->stmt_arena->is_conventional() &&
+      thd->mem_root != thd->stmt_arena->mem_root)
+    all_items= thd->stmt_arena->free_list;
+  else
+    all_items= thd->free_list;
+
+  mark_all_fields_used_in_query(thd, fields_info, &bitmap, all_items);
+
+  for (field_count=0; fields_info->field_name; fields_info++)
   {
     size_t field_name_length= strlen(fields_info->field_name);
     switch (fields_info->field_type) {
@@ -8182,26 +8243,44 @@ TABLE *create_schema_table(THD *thd, TABLE_LIST *table_list)
     case MYSQL_TYPE_MEDIUM_BLOB:
     case MYSQL_TYPE_LONG_BLOB:
     case MYSQL_TYPE_BLOB:
-      if (!(item= new (mem_root)
-            Item_blob(thd, fields_info->field_name,
-                      fields_info->field_length)))
+      if (bitmap_is_set(&bitmap, field_count))
       {
-        DBUG_RETURN(0);
+        if (!(item= new (mem_root)
+              Item_blob(thd, fields_info->field_name,
+                        fields_info->field_length)))
+        {
+          DBUG_RETURN(0);
+        }
+      }
+      else
+      {
+        if (!(item= new (mem_root)
+              Item_empty_string(thd, "", 0, cs)))
+        {
+          DBUG_RETURN(0);
+        }
+        item->set_name(thd, fields_info->field_name,
+                       field_name_length, cs);
       }
       break;
     default:
+    {
+      bool show_field;
       /* Don't let unimplemented types pass through. Could be a grave error. */
       DBUG_ASSERT(fields_info->field_type == MYSQL_TYPE_STRING);
 
+      show_field= bitmap_is_set(&bitmap, field_count);
       if (!(item= new (mem_root)
-            Item_empty_string(thd, "", fields_info->field_length, cs)))
+            Item_empty_string(thd, "",
+                              show_field ? fields_info->field_length : 0, cs)))
       {
         DBUG_RETURN(0);
       }
       item->set_name(thd, fields_info->field_name,
                      field_name_length, cs);
       break;
     }
+    }
     field_list.push_back(item, thd->mem_root);
     item->maybe_null= (fields_info->field_flags & MY_I_S_MAYBE_NULL);
     field_count++;

storage/innobase/handler/i_s.cc

@@ -294,7 +294,8 @@ field_store_index_name(
 	int	ret;
 
 	ut_ad(index_name != NULL);
-	ut_ad(field->real_type() == MYSQL_TYPE_VARCHAR);
+	ut_ad(field->real_type() == MYSQL_TYPE_VARCHAR ||
+              field->real_type() == MYSQL_TYPE_NULL);
 
 	/* Since TEMP_INDEX_PREFIX is not a valid UTF8, we need to convert
 	it to something else. */