BUG: Fixes explain operation

[xmnlab]

In this PR:

• Fixes explain operation

Resolves #1921
Depends on #1928

[codecov]

Codecov Report

Merging #1933 into master will decrease coverage by 1.21%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1933      +/-   ##
==========================================
- Coverage   87.46%   86.24%   -1.22%     
==========================================
  Files          89       93       +4     
  Lines       16405    17177     +772     
  Branches     2093     2244     +151     
==========================================
+ Hits        14349    14815     +466     
- Misses       1660     1958     +298     
- Partials      396      404       +8

Impacted Files	Coverage Δ
ibis/omniscidb/client.py	60.55% <100%> (+0.33%)	⬆️
ibis/bigquery/client.py	41.1% <0%> (-53.39%)	⬇️
ibis/bigquery/compiler.py	59.92% <0%> (-37.5%)	⬇️
ibis/bigquery/udf/api.py	80.48% <0%> (-14.64%)	⬇️
ibis/impala/compiler.py	91.23% <0%> (-5.2%)	⬇️
ibis/pandas/client.py	85.54% <0%> (-3.47%)	⬇️
ibis/bigquery/api.py	63.33% <0%> (-3.34%)	⬇️
ibis/expr/schema.py	89.77% <0%> (-1.14%)	⬇️
ibis/sql/compiler.py	94.26% <0%> (-0.39%)	⬇️
ibis/expr/api.py	92.33% <0%> (-0.37%)	⬇️
... and 9 more

[cpcloud]

LGTM! Thanks @xmnlab.