Right, I agree with the thoughts on the compatibility.
I think this might be a thing to ask solutions? 
>Четверг, 1 октября 2020, 14:38 +03:00 от Alexander Turenko <alexander.turenko@tarantool.org>:
> 
>> >How about lbox_key_def_merge() and underlying functions? I'm not sure
>> >they will work correct. At least I tried this on the branch:
>> >
>> > | tarantool> key_def = require('key_def')
>> > | tarantool> kd1 = key_def.new({{fieldno = 1, type = 'array'}})
>> > | tarantool> kd2 = key_def.new({{fieldno = 1, type = 'map'}})
>> > | tarantool> kd1:merge(kd2)
>> > | ---
>> > | - - type: array
>> > | is_nullable: false
>> > | fieldno: 1
>> > | ...
>> >
>> >It does not look correct.
>> This turns out to be the correct behavior:
>>  
>> tarantool> kd1 = key_def.new({{fieldno = 1, type = 'unsigned'}})
>> ---
>> ...
>> tarantool> kd2 = key_def.new({{fieldno = 1, type = 'string'}})
>> ---
>> ...
>> tarantool> kd1:merge(kd2)
>> ---
>> - - type: unsigned
>>     is_nullable: false
>>     fieldno: 1
>> …
>I'm not sure it is correct. But at least you proved that it is not in
>the scope of your current task :)
>
>An attempt to create conflicting indices gives an error, so I guess this
>case is not handled, because it was not possible until we exposed
>key_def directly to Lua.
>
>But another case makes me doubt even more:
>
> | tarantool> key_def = require('key_def')
> | tarantool> kd1 = key_def.new({{fieldno = 1, type = 'scalar'}})
> | tarantool> kd2 = key_def.new({{fieldno = 1, type = 'integer'}})
> | tarantool> kd1:merge(kd2)
> | ---
> | - - type: scalar
> | is_nullable: false
> | fieldno: 1
> | ...
> |
> | tarantool> kd1 = key_def.new({{fieldno = 1, type = 'integer'}})
> | tarantool> kd2 = key_def.new({{fieldno = 1, type = 'scalar'}})
> | tarantool> kd1:merge(kd2)
> | ---
> | - - type: integer
> | is_nullable: false
> | fieldno: 1
> | ...
>
>It would be natural to get most restricted type, when one type is subset
>of another. I assume the following invariant: for any given kd1 and kd2
>kd1:merge(kd2) should accept only those tuples that both kd1 and kd2
>accept (kd accepts a tuple when it is valid against kd).
>
>OTOH, the main use case for :merge() function is to compare tuples in
>the same way as a non-unique secondary index do (when a key parts of a
>primary index are implicitly merged into the secondary index ones). In
>this sense we can choose either subtype or supertype: it will not affect
>an order of tuples. So choosing of the first (secondary index's) key
>part looks okay.
>
>I don't know whether we should change anything or not, so just shared my
>findings and thoughts.
>
>If I'll somehow end with an idea that we should do something here, I'll
>file an issue.
>
>WBR, Alexander Turenko. 
 
 
--
Ilya Kosarev