Information Schema plugins: SHOW and FLUSH statements

Information Schema plugins can support SHOW and FLUSH statements.

SHOW

SHOW statements support is enabled automatically. A plugin only needs to specify column names for the SHOW statement in the old_name member of the field declaration structure. Columns with the old_name set to 0 will be hidden from the SHOW statement. If all columns are hidden, the SHOW statement will not work for this plugin.

Note that SHOW statement is a user-friendly shortcut; it's easier to type and should be easier to view — if the Information Schema table contains many columns, the SHOW statement is supposed to display only most important columns and fit nicely on the 80x25 terminal screen.

Consider an example, LOCALES plugin:

static ST_FIELD_INFO locale_info_locale_fields_info[]=
{
  {"ID", 4, MYSQL_TYPE_LONGLONG, 0, 0, "Id", 0},
  {"NAME", 255, MYSQL_TYPE_STRING, 0, 0, "Name", 0},
  {"DESCRIPTION", 255,  MYSQL_TYPE_STRING, 0, 0, "Description", 0},
  {"MAX_MONTH_NAME_LENGTH", 4, MYSQL_TYPE_LONGLONG, 0, 0, 0, 0},
  {"MAX_DAY_NAME_LENGTH", 4, MYSQL_TYPE_LONGLONG, 0, 0, 0, 0},
  {"DECIMAL_POINT", 2, MYSQL_TYPE_STRING, 0, 0, 0, 0},
  {"THOUSAND_SEP", 2, MYSQL_TYPE_STRING, 0, 0, 0, 0},
  {"ERROR_MESSAGE_LANGUAGE", 64, MYSQL_TYPE_STRING, 0, 0, "Error_Message_Language", 0},
  {0, 0, MYSQL_TYPE_STRING, 0, 0, 0, 0}
};

While the the INFORMATION_SCHEMA.LOCALES table has 8 columns, the SHOW LOCALES statement will only display 4 of them:

MariaDB [test]> show locales;
+-----+-------+-------------------------------------+------------------------+
| Id  | Name  | Description                         | Error_Message_Language |
+-----+-------+-------------------------------------+------------------------+
|   0 | en_US | English - United States             | english                |
|   1 | en_GB | English - United Kingdom            | english                |
|   2 | ja_JP | Japanese - Japan                    | japanese               |
|   3 | sv_SE | Swedish - Sweden                    | swedish                |
...

FLUSH

To support the FLUSH statement a plugin must declare the reset_table callback. For example, in the QUERY_RESPONSE_TIME plugin:

static int query_response_time_info_init(void *p)
{
  ST_SCHEMA_TABLE *i_s_query_response_time= (ST_SCHEMA_TABLE *) p;
  i_s_query_response_time->fields_info= query_response_time_fields_info;
  i_s_query_response_time->fill_table= query_response_time_fill;
  i_s_query_response_time->reset_table= query_response_time_flush;
  query_response_time_init();
  return 0;
}