Difference of query results in BigQuery Python Client Library

Question

I'd like to know the difference between query()'s return value and query().result()s'.

In BigQuery Python Client Library,

bigquery_client = bigquery.Client()
myQuery = "SELECT * FROM `mytable`"

## NOTE: This query result has just 1 row.
job = bigquery_client.query(myQuery)
for row in job:
    val1 = row

result = job.result()
for row in result:
    val2 = row

print(job == result) # False. I know QueryJob object is different to RowIterator object.
print(val1 == val2) # True

Why are val1 and val2 equivalent? Can the values be different for a very large query?

Answer 1

This is a yearlong self-answer.

Basically, 'job' and 'result' are different in my code.

bigquery_client.query() returns QueryJob instance. ( See https://googleapis.dev/python/bigquery/latest/generated/google.cloud.bigquery.client.Client.html#google.cloud.bigquery.client.Client.query )

But the QueryJob class has own _iter_ method and it returns iter(self.result()) ( See https://github.com/googleapis/python-bigquery/blob/main/google/cloud/bigquery/job/query.py#L1778 )

So 'job' becomes an iterator of result() for the for-in loop.

Thus, job != result but val1 == val2.