Adding back in lazy xr.Dataset and metadata parsing fix. (xqlsystems#94)

Author: alxmrs

xarray_sql/df.py

@@ -183,18 +183,15 @@ def read_xarray(ds: xr.Dataset, chunks: Chunks = None) -> pa.RecordBatchReader:
   Returns:
     A PyArrow Table, which is a table representation of the input Dataset.
   """
-  fst = next(iter(ds.values())).dims
-  assert all(
-      da.dims == fst for da in ds.values()
-  ), "All dimensions must be equal. Please filter data_vars in the Dataset."
-
-  blocks = list(block_slices(ds, chunks))
 
   def pivot_block(b: Block):
     return pivot(ds.isel(b))
 
-  schema = pa.Schema.from_pandas(pivot_block(blocks[0]))
-  last_schema = pa.Schema.from_pandas(pivot_block(blocks[-1]))
-  assert schema == last_schema, "Schemas must be consistent across blocks!"
+  fst = next(iter(ds.values())).dims
+  assert all(
+      da.dims == fst for da in ds.values()
+  ), "All dimensions must be equal. Please filter data_vars in the Dataset."
 
+  schema = _parse_schema(ds)
+  blocks = block_slices(ds, chunks)
   return from_map_batched(pivot_block, blocks, schema=schema)