As described in #7075, go-sql-driver/mysql v1.5.0 truncates timestamps
to microseconds, while v1.6.0 and above does not. That means upon
upgrading to v1.6.0, timestamps are written to the database with a
resolution of nanoseconds, and SELECT statements also use a resolution
of nanoseconds. We believe this is the cause of performance problems
we observed when upgrading to v1.6.0 and above.
To fix that, apply rounding in the application code. Rather than
just rounding to microseconds, round to seconds since that is the
resolution we care about. Using seconds rather than microseconds
may also allow some of our indexes to grow more slowly over time.
Note: this omits truncating some timestamps in CRL shard calculations,
since truncating those resulted in test failures that I'll follow up
on separately.
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