Hello, Sergie!

 I  tried to explore the source code from mi_create.c,ha_myisam.cc,sql/sql_table.cc,include/myisam.h,.and some other files.

The main task is to create  MI_UNIQUEDEF "uniques" for long unique constraints.

We have to consider all the cases where we need to create MI_UNIQUEDEF instead of MI_KEYDEF.

It will include queries like

           create table table1 (blob_column blob,unique(blob_column) );

           create table table1 (a int,blob_column blob,unique(a,blob_column) );

           create table table1 (a int,blob_column1 blob,blob_column2 blob,unique(blob_column1(300),blob_column2) );

(key with multiple blob columns and one of the blob column specified with prefix length).

I think we have to create  MI_UNIQUEDEF if any one of the columns in a key is a blob field without  prefix length.

In sql/sql_table, mysql_prepare_create_table is the function which prepares the table and key structures for table creation in mi_create. It generates an error if any one of the blob fields in a key is specified without length.

Currently, this task is limited to MyISAM, so if any other storage engine is selected, we have to generate the same error in  mysql_prepare_create_table.

In storage/myisam/ha_myisam.cc, table2myisam is a function which allocates and initializes myisam key and column definitions.The current function prototype of table2myisam is 

       table2myisam(TABLE  *table_arg, MI_KEYDEF  **keydef_out, MI_COLUMNDEF **recinfo_out, uint records_out)

We have to change it to

   table2myisam(TABLE  *table_arg,MI_KEYDEF  **keydef_out,MI_UNIQUEDEF ** uniquedef_out,MI_COLUMNDEF  **recinfo_out,uint records_out)

table2myisam initializes all the key definitions from table_arg->keyinfo.

So we can  set  a new flag  (say uniquedef) in a keyinfo struct in mysql_prepare_create_table if any one of the key_part consists of blob field without prefix length.

Later we can check the flag in table2myisam to see  if we want to create MI_KEYDEF or MI_UNIQUEDEF.

Thanks,

Shubham.

On 5 March 2016 at 03:52, Sergei Golubchik <serg@mariadb.org> wrote:

Hi, Shubham!

On Mar 03, Shubham Barai wrote:
>
> I am interested in the project "Unique indexes for blobs".I have read the
> description of the project given on the link
> https://jira.mariadb.org/browse/MDEV-371.

Great!

> I want to know what exactly is the task for this project.Any help
> would be greatly appreciated

See storage/myisam/mi_create.c. You see that a myisam table can have
keys (defined in MI_KEYDEF structure) and so-called "uniques" (defined
by MI_UNIQUEDEF).

Keys have length limitation, keys can be unique (HA_NOSAME flag) or not
unique.

"uniques" have no length limitation.

When one creates a unique key from SQL:

  CREATE TABLE ... (... UNIQUE KEY (...) ... )

this will always create a key (MI_KEYDEF) in a MyISAM table. That's why
unique constraints in MyISAM have a limited length.

This task is about creating MI_UNIQUEDEF "uniques" instead of MI_KEYDEF
"keys" for long unique constraints.

Regards,
Sergei
Chief Architect MariaDB
and security@mariadb.org

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