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Re: GSoC 2016:Unique indexes for blobs


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)


> 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
> Thanks,
> Shubham.

Very good!

Chief Architect MariaDB
and security@xxxxxxxxxxx

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