Bug #16869534 QUERYING SUBSET OF COLUMNS DOESN'T USE TABLE CACHE; OPENED_TABLES I

Description: When querying a subset of columns from the information_schema.TABLES

Analysis: When information about tables is collected for statements like
"SELECT ENGINE FROM I_S.TABLES" we do not perform full-blown table opens
in SE, instead we only use information from table shares from the Table
Definition Cache or .FRMs. Still in order to simplify I_S implementation
mock TABLE objects are created from TABLE_SHARE during this process.
This is done by calling open_table_from_share() function with special
arguments. Since this function always increments "Opened_tables" counter,
calls to it can be mistakingly interpreted as full-blown table opens in SE.

Note that claim that "'SELECT ENGINE FROM I_S.TABLES' statement doesn't
use Table Cache" is nevertheless factually correct. But it misses the
point, since such statements a) don't use full-blown TABLE objects and
therefore don't do table opens b) still use Table Definition Cache.

Fix: We are now incrementing the counter when db_stat(i.e open flags for ha_open(

we have considered an optimization which would use TABLE objects from
Table Cache when available instead of constructing mock TABLE objects,
but found it too intrusive for stable releases.

mysql-test/r/create.result

@@ -291,7 +291,7 @@ Level	Code	Message
 Note	1050	Table 't1' already exists
 show status like "Opened_tables";
 Variable_name	Value
-Opened_tables	2
+Opened_tables	1
 select * from t1;
 a	b
 1	1