[BUG] Performance degradation due defer about mydumper HOT 7 OPEN

Hi @midenok,

I assigned this to you as you implemented this feature request. You might be able to explain the performance degradation.

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The above example has only one index, integer key. Defer cannot help here, but OTOH has negative performance impact due to extra queuing. Maybe it should be disabled by default and enabled only by command line option.

from mydumper.

@midenok, Can you explain in which cases it will be useful?
I'm thinking to automatically set some variables like --rows, and this might be one of them.

from mydumper.

I hope commit message explains it:

    Improve mydumper parallelism: integer sharding deferrer

    Integer PK is the only PK that allows parallelized sharding on
    mydumper. Non-integer PK tables utilize only one thread per table and
    if they are going in the end such tables cause some worker threads to
    be idle. To mitigate such cases we enqueue integer tables only after
    all non-integer tables processed.

from mydumper.

Hi @midenok, what you are explaining is not what I saw. I ended up in a scenario where a integer pk was being split by a fixed amount of rows.

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