Hi Oleksandr!! :)

Hi, Roberto!
at "value" parameter, COLUMN_CREATE know that we are using a STRING, but should be any other data type

The *|as type|* part allows one to specify the value type. In most cases, this is redundant because MariaDB will be able to deduce the type of the value. Explicit type specification may be needed when the type of the value is not apparent. For example, a literal |'2012-12-01'| has a CHAR type by default, one will need to specify|'2012-12-01' AS DATE| to have it stored as a date. See the Datatypes <https://mariadb.com/kb/en/mariadb/dynamic-columns/#Datatypes> section for further details.

You are wrong, with UDF function result type known because then goes something like:

hum nice,

but COLUMN_CREATE should return *always* string data type right? and the second parameter of function, in this case "value", is a parameter with a value="value" and data type=string , right?

i'm not a internal expert, i'm trying to understand some doubts, check i'm my affirmations are right:

1) UPDATE/DELETE/SELECT, at WHERE "part", data type is a problem to operators for example "=", cause it need a cast before compare... for example 3="03", it must know if we will convert interger to string, or string to integer, that's why a CAST at COLUMN_GET is important

2) at SELECT "part" data type is a problem to mysql protocol, cause we must send to client what type each column will receive, at this part i don't know how COLUMN_GET should work... internally it return the data type before receiving data? or it will read some data and after inform the data type being used? or each row it return a data type, and internally mysql buffer the SELECT result, and identify after SELECT what data type should be used?

3) at UPDATE/INSERT/REPLACE data type is important to storage don't truncate,overflow or any other possible data loss

mysql>*|CREATE FUNCTION metaphon RETURNS STRING SONAME 'udf_example.so';|*
mysql>*|CREATE FUNCTION myfunc_double RETURNS REAL SONAME 'udf_example.so';|*
mysql>*|CREATE FUNCTION myfunc_int RETURNS INTEGER SONAME 'udf_example.so';|*

hum, if we have a function that could return INT or STRING or REAL, we must have 3 functions, right? i'm trying to search at mariadb source code how COLUMN_GET is implemented, should a internall function return >1 datatypes and UDF not? (i'm begginer to internall functions)

( http://dev.mysql.com/doc/refman/5.1/en/udf-compiling.html )

Type just have to be sent to the client. Some functions required argument of certain type so ask correcpondent val*() method of Item object but top most function (or constant or field) determinate type of the column in SELECT list and the type should be known.

nice, in this case

CONCAT('abc',any_function())

the any_function could return any data type, but client/server must know what datatype CONCAT return, right?

So previous statement about 3rd column was very precise.

your comments make things easier to understand each day :) many thanks