yes, this was discussed for quite a while, and has a long history in MySQL bug database.
there was even an attempt to implement this, but somehow it failed, don't really know why.
I guess, we can give it another try.

humm, nice =)
well i think the problem was sending big quanty of bytes in/out database, but... maybe a function to help index could be nice... i think that many guys implement something like
select count(*) from files where some_hash_field=(length || ';' || some_hash_value_calculated_in_a_script)
if count(*)> 0
select * from files where some_hash_field=(length || ';' || some_hash_value_calculated_in_a_script)
well, the first part is realy nice... but if it return >1 well send file is nicer than read many files with same size...

i don´t know if it could be nice, but implementation is close to hash with any binary char field, since it´s a hash not a btree... (i´m wrong?)
well =) let´s see what happen =)
it´s a feature request, don´t think that´s really needed but a 'formal' or a 'recommended' way to check if a 'file' (blob) is in database could be good, at least in documentation...

hi sergey any idea where the patch about this try was saved? i want to see if it's easy to implement but i don't know how to start

No, I don't. Any anyway, if I would like to do it, I would rather start
from scratch, than from some old incomplete patch.

ok, at least a hash index/unique index could help a lot

this should be done in each storage engine, or index is a general "feature" of mariadb?

in each storage engine. in particular, MyISAM and Aria almost support this already, and that "attempt" that I was referring to was exactly about making them support it fully.

hummm nice, in first step make myisam and aria blob index possible, innodb and others is a second step, right?

Hi Sergei,

I am interested in working on this feature as part of GSoC. From Google search it seems that this feature is really important as many people report problem related to having BLOB/TEXT as primary key or creating index for it.

I have good experience with databases but currently I am new to mariadb source code. I would appreciate if you can help me in getting started.

Thanks

How does this handle COLLATION of a large TEXT field? Is there a way to "hash" while honoring complex UTF-* case and accent handling?

It should do that automatically. But that part has a bug, though so it doesn't always work. Reported as MDEV-27653

For the record, the following patch should disable the MDEV-371 functionality:

diff --git a/sql/sql_table.cc b/sql/sql_table.cc

index 6e8a4795f21..f8f3eefc114 100644

--- a/sql/sql_table.cc

+++ b/sql/sql_table.cc

@@ -2441,6 +2441,8 @@ static inline void make_long_hash_field_name(LEX_CSTRING *buf, uint num)

 static Create_field * add_hash_field(THD * thd, List<Create_field> *create_list,

                                       KEY *key_info)

 {

+  my_error(ER_TOO_LONG_KEY, MYF(0), 1000);

+  return nullptr;

   List_iterator<Create_field> it(*create_list);

   Create_field *dup_field, *cf= new (thd->mem_root) Create_field();

   cf->flags|= UNSIGNED_FLAG | LONG_UNIQUE_HASH_FIELD;

This may be useful for testing, because there are many open bugs related to indexed virtual columns, and MDEV-371 is internally creating hidden indexed virtual columns.