maria-developers team mailing list archive

[Shubham Barai <shubhambaraiss@xxxxxxxxx>]

Hello Sergei,

            As you suggested, I tried  to develop a prototype  just for
blobs.
So far it is working fine but I am still verifying  it for different  cases.
I will provide you the link to my Github repository shortly.

Thanks,
Shubham

On 16 March 2016 at 17:55, Sergei Golubchik <serg@xxxxxxxxxxx> wrote:

> Hi, Shubham!
>
> On Mar 16, Shubham Barai wrote:
> > 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.
>
> Yes. And also for any other UNIQUE constraint that is too long for a
> normal index. For example, many long VARCHAR columns.
>
> But, of course, for a prototype one can start just with blobs.
>
> > 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.
>
> There's a generic check whether the key is too long. It can be used
> here, it doesn't depend on the storage engine.
>
> > 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)
>
> Right.
>
> > 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.
>
> There's a field 'algorithm' already. Because MI_UNIQUEDEF in MyISAM is,
> basically, an index of hashed column values, it is kind of a hash index.
> So you can use algorithm=HA_KEY_ALG_HASH to mark such columns. And the
> user will be able to create these indexes explicitly with
>
>   create table table1 (blob_column blob,unique(blob_column) using hash);
>
> > Later we can check the flag in table2myisam to see  if we want to
> > create MI_KEYDEF or MI_UNIQUEDEF.
> >
> > Thanks,
> > Shubham.
>
> Very good!
>
> Regards,
> Sergei
> Chief Architect MariaDB
> and security@xxxxxxxxxxx
>