add lock clause support in select queries

[deoxxa]

No description provided.

[coveralls]

Coverage decreased (-0.02%) to 93.71% when pulling 10ad98f on deoxxa:add-lock-clause into 7f381db on umisama:master.

[umisama]

I'm sorry to have got to wait for you because I miss this PR.

LOCK clause is supported by only PostgresSQL. Can you write this to godoc clearly?

[yosida95]

@umisama LOCK clause (row level lock) is not supported by only PostgreSQL but also MySQL (InnoDB engine).

However this patch won't work in MySQL because MySQL uses SELECT ... LOCK IN SHARE MODE clause to represent shared lock, like described in https://dev.mysql.com/doc/refman/5.7/en/innodb-locking-reads.html.

[umisama]

thank you yosida95. I've checked LOCK clause is supported by InnoDB but it is not standard SQL.
I want to compatible around DB engines as much as possible.

[deoxxa]

Perhaps it would be better if the functions were named PostgresLocks and PostgresLock? That way it'd be very clear that they only work in Postgres.

[umisama]

Hmm, I hope using dialect to compatible with mysql.

OK! I'll implement LOCK clause with your idea!
Please wait few days!