maria-developers team mailing list archive

[Shubham Barai <shubhambaraiss@xxxxxxxxx>]

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@xxxxxxxxxxx> 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@xxxxxxxxxxx
>