Do we need to SELECT 1 before each query?

With the recent clustering release, we’re looking at whether we can use BQ for queries which previously required an indexed DB.

For these, the overhead is more important than the throughput. The overhead is high - in the case that the query is cached (if not, add 1s to all these, for a 700 row query on a small cluster):

• 73ms: query time reported by BQ
• 1160ms: query time logged by pandas-gbq
• 3100ms: wall time reported by %timeit -n 1 -r 1 gbq.read_gbq(query)

The latter two are particularly far apart. One of the reasons for this is that we issue a SELECT 1 query before each query the user submits. This happens as part of _try_credentials: https://github.com/pydata/pandas-gbq/blob/e753cc4e61b0089dcea6330eb8c77e93a62984a7/pandas_gbq/auth.py#L281

Is this required? Could we instead attempt the query, and only attempt different auth methods if there’s a failure?

[tswast]

Also in #161, I’d like to cache the credentials after the first call in a global Context object to avoid subsequent unnecessary auth calls.

[max-sixty]

I think we can close this - now it’s a single call per session?