Hi!
> So, a few questions as to how MySQL does it, and hopefully a discussion on
I can say what I have seen in MySQL 5.1 - most notably sql/sql_show.cc
> how Drizzle should do it:
> 1) Are the information_schema tables views or temporary tables?
As far as I know they are not "views" in the sense of named schema
objects defined by a query expression.
They are temporary tables in the sense that they are transient
in-memory structures that are bound to the thread (the current
connection).
In C they are defined through a special struct in table.h
typedef struct st_schema_table
{
const char* table_name;
ST_FIELD_INFO *fields_info;
/* Create information_schema table */
TABLE *(*create_table) (THD *thd, TABLE_LIST *table_list);
/* Fill table with data */
int (*fill_table) (THD *thd, TABLE_LIST *tables, COND *cond);
/* Handle fileds for old SHOW */
int (*old_format) (THD *thd, struct st_schema_table *schema_table);
int (*process_table) (THD *thd, TABLE_LIST *tables, TABLE *table,
bool res, LEX_STRING *db_name, LEX_STRING *table_name);
int idx_field1, idx_field2;
bool hidden;
uint i_s_requested_object; /* the object we need to open(TABLE | VIEW) */
} ST_SCHEMA_TABLE;
I don't think this is the same as a table you create with CREATE TEMP TABLE.
> 2) Where are the definitions stored for the information_schema tables?
If you mean, the column layout that is defined as a list in the C code.
The member
ST_FIELD_INFO *fields_info;
of ST_SCHEMA_TABLE holds this list.
The specific IS table layouts are hardwired in sql_show.cc
For example, here is the column layout for I_S.processlist:
ST_FIELD_INFO processlist_fields_info[]=
{
{"ID", 4, MYSQL_TYPE_LONGLONG, 0, 0, "Id", SKIP_OPEN_TABLE},
{"USER", 16, MYSQL_TYPE_STRING, 0, 0, "User", SKIP_OPEN_TABLE},
{"HOST", LIST_PROCESS_HOST_LEN, MYSQL_TYPE_STRING, 0, 0, "Host",
SKIP_OPEN_TABLE},
{"DB", NAME_CHAR_LEN, MYSQL_TYPE_STRING, 0, 1, "Db", SKIP_OPEN_TABLE},
{"COMMAND", 16, MYSQL_TYPE_STRING, 0, 0, "Command", SKIP_OPEN_TABLE},
{"TIME", 7, MYSQL_TYPE_LONGLONG, 0, 0, "Time", SKIP_OPEN_TABLE},
{"STATE", 64, MYSQL_TYPE_STRING, 0, 1, "State", SKIP_OPEN_TABLE},
{"INFO", PROCESS_LIST_INFO_WIDTH, MYSQL_TYPE_STRING, 0, 1, "Info",
SKIP_OPEN_TABLE},
{0, 0, MYSQL_TYPE_STRING, 0, 0, 0, SKIP_OPEN_TABLE}
};
> 3) How and when do the definitions get loaded?
If you mean by "definition" the column layout, then I think it is
loaded from the start on. The arrays of ST_FIELD_INFO are global
variables.
I am not entirely sure how and when the data is loaded.
> I see these questions (particularly #3) as important particularly for
> plugins, as some plugins will (heck, I'd probably argue that most of the
> major ones *should*) add to the information_schema database.
well, I believe that for information schema plugins, the definition is
loaded at least upon INSTALL plugin, but also on server startup (for
those plugins already installed)
I hope this helps,
Roland
>
> --
> - Sheeri K. Cabral
>
>
>
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>
--
Roland Bouman
http://rpbouman.blogspot.com/
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