fix(oracle): support NTH_VALUE FROM FIRST and LAST

[aosingh]

Summary

Fix Oracle parsing and generation of the optional FROM FIRST / FROM LAST modifier for the NTH_VALUE analytic function.

Oracle supports this syntax:

SELECT NTH_VALUE(1, 1) 
FROM FIRST OVER (ORDER BY 1) AS FIRST_VALUE
FROM DUAL

Before

SQLGlot failed to parse the statement:

File "/project/sqlglot/parser.py", line 2136, in parse
  File "/project/sqlglot/parser.py", line 2256, in _parse
  File "/project/sqlglot/parser.py", line 2223, in _parse_batch_statements
  File "/project/sqlglot/parser.py", line 2386, in _parse_statement
  File "/project/sqlglot/parser.py", line 3991, in _parse_select
  File "/project/sqlglot/parser.py", line 4116, in _parse_select_query
  File "/project/sqlglot/parser.py", line 4452, in _parse_from
  File "/project/sqlglot/parser.py", line 5080, in _parse_table
  File "/project/sqlglot/parser.py", line 4275, in _parse_table_alias
  File "/project/sqlglot/parser.py", line 9707, in _match_r_paren
  File "/project/sqlglot/parser.py", line 2093, in raise_error
sqlglot.errors.ParseError: Expecting ). Line 1, Col: 50.
  SELECT NTH_VALUE(1, 1) FROM FIRST OVER (ORDER BY 1) AS FIRST_VALUE FROM DUAL

After

The Oracle parser recognizes FROM FIRST and FROM LAST immediately after NTH_VALUE(...) and stores the modifier on the existing exp.NthValue.from_first AST argument. The Oracle generator emits the modifier before the OVER clause, preserving the Oracle syntax:

>> sqlglot.parse_one("SELECT NTH_VALUE(1, 1) FROM FIRST OVER (ORDER BY 1) AS FIRST_VALUE FROM DUAL", dialect="oracle")

Select(
  expressions=[
    Alias(
      this=Window(
        this=NthValue(
          this=Literal(this=1, is_string=False),
          offset=Literal(this=1, is_string=False),
          from_first=True),
        order=Order(
          expressions=[
            Ordered(
              this=Literal(this=1, is_string=False),
              nulls_first=False)]),
        over=OVER,
        join_mark=False),
      alias=Identifier(this=FIRST_VALUE, quoted=False))],
  from_=From(
    this=Table(
      this=Identifier(this=DUAL, quoted=False))))

Tests

  python -m unittest tests.dialects.test_oracle
  make test
  make testc

All passed.