convert select() to no-op if the current database is the same as target one

[DRVTiny]

select() with the same database number as the already selected one leads to no exchange with the Redis server

[stefanwille]

Hi @DRVTiny, thanks for your PR.

To me this looks like an optimization for a case that has little practical performance impact?

[DRVTiny]

I don't think so. For example, if you store different kind of serialized objects in different databases - you either have to use N redis connections or 1 connection but with frequently select's between databases. In common case you have to compare database property of redis connector and number of target database manually in your - or crystal-redis driver can do this for you - maybe in some more sophisticated way. select() is not too time consuming operation in 99% of scenarious, but in library code it will be reasonable to think about rest 1%, because sockets may be slow and excessive commands sensed through when you can avoid this - may leads to additional delays without any reasons: it is very simple to check whether you need to switch from one database to another or no. So why not to do this? I don't think that using one redis connector in several parallel threads will be more common scenario than those i've describer earlier... пн, 25 июня 2018 г., 20:06 Stefan Wille <notifications@github.com>:

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Hi @DRVTiny <https://github.com/DRVTiny>, thanks for your PR. To me this looks like an optimization for a case that has little practical performance impact? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#66 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AB7q9xx97W2LIT1xmwLikL3I4TbbwQ0iks5uARiQgaJpZM4U2Y5v> .

[DRVTiny]

stefanwille, this pull request includes setting database as Redis instance attribute as well.
Because of some unobvious bugs in Redis driver code, i've got, for example, connections in Redis::PooledClient pool where select() was not called for some very strange reasons. To debug this case i need to know current @database value... somehow.
It is normal practice to store different kind of Redis objects in different databases. This possibility is a basic Redis functional, so i need to be possible to use it in my Crystal code.

[stefanwille]

Merged, thanks!