presto-parser/src/main/java/io/prestosql/sql/parser/AstBuilder.java/AstBuilder/visitSampledRelation
class AstBuilder:    @Override
    public Node visitSampledRelation(SqlBaseParser.SampledRelationContext context)
    {
        Relation child = (Relation) visit(context.aliasedRelation());

        if (context.TABLESAMPLE() == null) {
            return child;
        }

        return new SampledRelation(
                getLocation(context),
                child,
                getSamplingMethod((Token) context.sampleType().getChild(0).getPayload()),
                (Expression) visit(context.percentage));
    }