SQL-to-RelNode conversion is slow if table contains hundreds of fields

When converting from Sql To Rel, In SqlToRelConverter for every single instance of an identifier we create a new map in 

org.apache.calcite.sql2rel.SqlToRelConverter.Blackboard#lookupExp

final Map<String, Integer> fieldOffsets = new HashMap<>();
for (RelDataTypeField f : resolve.rowType().getFieldList()) {
if (!fieldOffsets.containsKey(f.getName())) {
fieldOffsets.put(f.getName(), f.getIndex());
}
}
final Map<String, Integer> map = ImmutableMap.copyOf(fieldOffsets);

So for a Sql Query

SELECT name, nation FROM customer

We would do the above operation twice.

Memoization of this information will improve performance.

In my database, I had observed that for a large table involving 1200 columns and a huge select having multiple expressions and operators, this part was a bottleneck.