Update chunker requirements

[bjk-soundcloud]

The chunker does not require an auto_increment column, it simply
requires an integer column in order to find the start/limit of the
table.

Relax the migrator/table checks to only look for an id INT column.

• Add support for BIGINT id columns
• Add fixture for BIGINT id column
• Add fixture for "broken" varchar id column
• Update specs

[bjk-soundcloud]

@hannestyden There are still some specs to fix, but I wanted to get this out early for review.

[grobie]

While it's true that the chunker does not strictly require an auto_increment column, it's implemented on the assumption that the identifier column is monotonically increasing. AFAIK the chunker behaves quite poorly in case of huge gaps between rows.

I'd suggest to add a note to the README explaining that a monotonically increasing counter is recommended / warning that a migration will be quite slow with column of different value distributions.

[bjk-soundcloud]

The original version of LHM did not require auto_increment. This requirement was added at the time we added support for id as a surrogate key. But it breaks LHM for things like the "purgatory" tables where they are a mirror of a schema, but have sparse data that was removed from their mirror.

The real issue is when the id data is sparse. I would like to re-write the chunker to use pt-online-schema-change style linked pagination. This would solve a the sparseness issue.

I can add a note about sparse id issues.

[bjk-soundcloud]

@arthurnn @hannestyden Ok, all the tests pass.

[bjk-soundcloud]

Ping, @hannestyden. 😄

[bjk-soundcloud]

@grobie How does this look to you now?

[arthurnn]

why this is in the middle? (can you move it to the top? )

[hannestyden]

Looking good! 👍

[janogonzalez]

👍