pandas gets stuck when trying to read from bigquery

Question

I have a fairly large table in big query ( app. 9M rows) and I would like to read it via pandas.

I've tried reading and using the [pd.read_gbq()][1] function, which works fine on small tables.

On the large table it gets stuck after 50 secs or so (logs show elapsed .. 50s) - without giving an error or anything.

My question is how can I read that table using pd (chunks?). Any conventions on scaling up these bigquery reads will be helpful.

EDIT / resolution

adding to Khan's answer, I ended up implementing chunks, writing 500,000 each time to a file, then reading these files to dataframe like so:

def download_gbq_table(self):
    if not os.path.exists(self.tmp_dir):
        os.makedirs(self.tmp_dir)
    increment = 100000

    intervals = list(range(0, self.table_size, 100000))
    intervals.append(self.table_size - intervals[len(intervals)-1])

    df = pd.DataFrame()

    for offset in intervals:
        query = f"select * from `<table_name>` limit {increment} offset {offset};"
        logger.info(f"running query: {query}")
        start_time = time.time()
        tmp_df = pd.read_gbq(query,
                       project_id=self.connection_parameters['project_id'],
                       private_key=self.connection_parameters['service_account'],
                       dialect='standard'
                        )
        df = pd.concat([df, tmp_df])
        logger.info(f'time took: {str(round(time.time() - start_time, 2))}')
        if len(df) % 500000 == 0:
            df.to_csv(os.path.join(self.tmp_dir, f'df_{str(offset + increment)}.csv'))
            df = pd.DataFrame()

def read_df_from_multi_csv(self):
    all_files = glob.glob(os.path.join(self.tmp_dir, "df_*"))
    df_list = []

    for f in all_files:
        start_time = time.time()
        df_list.append(pd.read_csv(f))
        logger.info(f'time took for reading {f}: {str(round(time.time() - start_time, 2))}')

    return pd.concat((pd.read_csv(f) for f in all_files))

Answer 1

Pandas' read_gbq function currently does not provide a chunksize parameter (even though its opposite to_gbq function does provide a chunksize parameter).

Anyways, you can solve your problem with adding LIMIT and OFFSET to your SQL query read stuff iteratively from BigQuery. Something on the lines of:

project_id = "xxxxxxxx"

increment=100000
chunks=[*range(0, 9000000, 100000)]

chunks[-1]+=increment 
intervals=[[chunks[i-1], chunks[i]+1] for i, e in enumerate(chunks) if i > 0]

query_str="select * from `mydataset.mytable` limit {end} offset {start};"

for start, end in intervals:
   query = query_str.format(start=start, end=end)
   df = pd.read_gbq(query, project_id)
   #-- do stuff with your df here..