Loosen anchor restriction.

[ueshin]

Trying to loosen anchor restriction.

When the underlying Spark DataFrame is the same and index metadata is not modified, we can work without joins.

[codecov-commenter]

Codecov Report

Merging #1522 into master will decrease coverage by 0.02%.
The diff coverage is 89.28%.

@@            Coverage Diff             @@
##           master    #1522      +/-   ##
==========================================
- Coverage   94.02%   93.99%   -0.03%     
==========================================
  Files          36       36              
  Lines        8479     8489      +10     
==========================================
+ Hits         7972     7979       +7     
- Misses        507      510       +3     

Impacted Files	Coverage Δ
databricks/koalas/frame.py	95.47% <85.00%> (-0.12%)	⬇️
databricks/koalas/groupby.py	90.30% <100.00%> (+0.03%)	⬆️
databricks/koalas/utils.py	96.31% <100.00%> (-0.07%)	⬇️

---

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 9f004ce...a818316. Read the comment docs.