Results of this change look good in many aspects.

Code change made summary

The old code made a really hard job on every INFORMATION_SCHEMA query:

• converted every ST_FIELD_INFO into Item
• then converted every new Item to Field
• then put every new Field into List<Field>
• and only after that called create_tmp_table() with the List<Field>

The new code just creates a table directly from the array of ST_FIELD_INFO without such heavy conversion

Unnecessary classes removed

These classes were used only in I_S table creation, in the ST_FIELD_INFO->Item->Field chain:

• Item_return_date_time
• Item_blob

After this change these classes were not needed any more, so they were removed.
Note, it should be also easy to remove class Item_return_int (in a separate change).

Creation of INFORMATION_SCHEMA tables is now faster

I_S tables are now created directly from an array of ST_FIELD_INFO, no such heavy conversion involved any more.

This example code runs 1% faster after the change:

mysql --socket=/tmp/mysql.sock -s --database=test << QEND

DELIMITER $$

CREATE OR REPLACE PROCEDURE p1()

BEGIN

  FOR i IN 1..1000000

  DO

    BEGIN

      SELECT * FROM INFORMATION_SCHEMA.COLUMNS LIMIT 0;

    END;

  END FOR;

END;

$$

DELIMITER ;

QEND

time mysql --socket=/tmp/mysql.sock -s --database=test --execute="CALL p1()"

The code is now data type friendly

I_S columns of new data types now work out of the box, without a need of new wrapper classes like the former Item_blob, Item_return_date_time or Item_return_int. A new data type developer just needs to override Type_handler_xxx::create_schema_field(), which is one line method with a single new Field_xxx() statement.

• User defined plugins are now able to have fields of their own data type in their pluggable INFORMATION_SCHEMA tables.
• We'll be adding built-in SQL standard data types soon, such as TIMESTAMP WITH TIME ZONE, INTERVAL, so adding I_S columns of these types is also trivial.

It's now easy to extend creating of temporary tables

The new class Create_tmp_table now understands two input data formats:

• List<Item>
• ST_SCHEMA_TABLE

Adding new formats is now trivial and does not need heavy conversion like ST_FIELD_INFO->Item->Field. One just needs to add a proper add_fields() method.

Examples:

• it's now trivial to create a temporary table directly from List<Item>. Currently we convert List<Item> into List<Field> in such cases. Direct table creation from List<Item> can make the code faster.
• On ALTER, we convert every Field into Create_field. Such conversion is not really needed. We could create an altered table from a mixture of old Fields with new Create_fields, which would improve performance.