fix(sqla): avoid unnecessary groupby in samples request

[villebro]

SUMMARY

When requesting samples from a dataset, a GROUP BY was incorrectly added to the query due to the metrics parameter in the QueryObject being set to an empty list. This adds unnecessary computational expense to the query and doesn't reflect the raw data correctly by hiding potential duplicates in the underlying data. This PR fixes the bug, adds assertions to existing tests to ensure the sample query is executed without a GROUP BY clause and adds instructions to the API spec on what the difference is between a null vs empty array for the metrics parameter.

TESTING INSTRUCTIONS

1. Go to explore and open the Network tab in the Chrome debugger
2. Click on "Samples"
3. Notice that the query in the chart data request previously included a GROUP BY clause.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #18579 (36eceb0) into master (8b0634c) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #18579      +/-   ##
==========================================
- Coverage   66.29%   66.29%   -0.01%     
==========================================
  Files        1594     1594              
  Lines       62623    62623              
  Branches     6312     6312              
==========================================
- Hits        41518    41516       -2     
- Misses      19456    19458       +2     
  Partials     1649     1649              

Flag	Coverage Δ
hive	52.07% <100.00%> (-0.07%)	⬇️
mysql	81.24% <100.00%> (-0.01%)	⬇️
postgres	81.29% <100.00%> (-0.01%)	⬇️
presto	51.91% <100.00%> (-0.07%)	⬇️
python	81.72% <100.00%> (-0.01%)	⬇️
sqlite	80.99% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/charts/schemas.py	99.34% <ø> (ø)
superset/common/query_actions.py	93.05% <100.00%> (ø)
superset/connectors/sqla/models.py	88.29% <0.00%> (-0.23%)	⬇️

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[amitmiran137]

LGTM ! nice to feel the value of chart data separated test suite!

[villebro]

LGTM ! nice to feel the value of chart data separated test suite!

Agreed, the chart data test suite is a great tool to help harden the poorly tested sqla connector 👍