Add always false filter to queries to improve speed (particularly on Snowflake)

[DylanBaker]

We run limit 0 queries in order to test the validity of the SQL Looker generates. On lots of warehouses, that processes no data and is very fast (BigQuery and Redshift, notably).

On Snowflake, it still processes a lot of data and there isn't an EXPLAIN functionality we can use right now. The proposed solution is to add a 1=2 filter to all queries, which does stop it from processing data.

[JarredLHumphrey]

Just an FYI that you can see the query planner output via the Snowflake instances UI at:
https://<snowflakehost>.com/console#/monitoring/queries

Once on that page, click the query id you want the plan for, and then click the Profile tab.

I took the liberty to run two queries to show the difference in output for LIMIT 0 vs WHERE 1=2.

Query: SELECT * FROM INFORMATION_SCHEMA.COLUMNS LIMIT 0;
Plan:

Query: SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE 1=2 LIMIT 0;
Plan:

This demonstrates that the filtering is done as part of the initial generator step so less data is fetched to the instance before the limit is applied.

[codecov]

Codecov Report

Merging #129 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #129   +/-   ##
=======================================
  Coverage   66.52%   66.52%           
=======================================
  Files           9        9           
  Lines         684      684           
=======================================
  Hits          455      455           
  Misses        229      229

Impacted Files	Coverage Δ
spectacles/client.py	57.03% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7d1f5de...8ea13fd. Read the comment docs.