At hotglue, we power thousands of data syncs every day — improving the performance of these syncs translates into huge cost savings and better experiences for our users. Our customers count on us to deliver connectors that are reliable, fast, and ensure high data quality.

In this post, we’ll walk through how our team optimized our Microsoft SQL target and significantly improved throughput. We’ll compare three approaches — pymssql, pyodbc, and bcp — and share the results of our testing.

Let’s dive in!

Connectivity issues with pymssql

Our original Microsoft SQL Server target was built using SQLAlchemy with pymssql.

An AI startup was trialing our product, and their use case centered around reading large amounts of data from CRMs like HubSpot and Salesforce and writing it to their Azure SQL server.

The first issue we encountered was with connecting to Azure SQL Server. It turns out this was a common issue with pymssql, as Azure SQL Server requires encryption and the pymssql wheel does not have SSL bindings by default (there’s a nice StackOverflow thread about this).

Switching to pyodbc

Rather than fiddle with installing pymssql from source, we switched to using pyodbc with the official Microsoft SQL Server driver. This was easy enough as we were already using SQLAlchemy – just changing our connection URL and installing the ODBC driver was enough:

connection_url = sqlalchemy.engine.url.URL.create(
    drivername="mssql+pyodbc",
    username=config['user'],
    password=urllib.parse.quote_plus(config["password"]),
    host=config["host"],
    port=config["port"],
    database=config["database"],
    query={
        "driver": "ODBC Driver 17 for SQL Server",  # Use Microsoft's ODBC driver
        "Encrypt": "yes",  # Ensures SSL encryption for Azure SQL
        "TrustServerCertificate": "yes",  # Prevents bypassing certificate validation
        "MARS_Connection": "Yes",
        "ConnectRetryCount": "3",
        "ConnectRetryInterval": "15"
    }
)

This solved the initial issue and we were able to connect to Azure SQL Server! However, we immediately ran into performance issues – writing even relatively small amounts of records (we tested with batches of 10,000) was slow. Worse, we seemed to have intermittent timeout issues while trying to write data, with error messages like:

[08S01] [Microsoft][ODBC Driver 17 for SQL Server]TCP Provider: Error code 0x68 (104) (SQLExecDirectW)

Trying to boost pyodbc performance

Doing some research we found that other folks had hit similar performance constraints with pyodbc – the recommendation was to try using the fast_executemany=True flag. Again, this was straightforward thanks to SQLAlchemy:

engine = sqlalchemy.create_engine(
    self.sqlalchemy_url, 
    fast_executemany=True,
    echo=False,
    isolation_level=None
)

However, this broke our core logic as our target initially creates local temporary tables (denoted in MS SQL using a #). The problem is that fast_executemany breaks with local temp tables – that rabbit hole took us to a GitHub issue thread on the pyodbc repo discussing possible solutions.

According to that thread, we could have avoided this error by adding the flag UseFMTONLY or switching to global temp tables. However, we were still not convinced this fast_executemany flag would solve our core performance issue, so we wrote a simple workaround to create “fake” temp tables prefixed with TEMP_

So we dropped the original temp table creation logic:

DROP TABLE IF EXISTS #{from_table_name};

SELECT TOP 0 *
    into #{from_table_name}
    FROM {from_table_name};

and replaced it with creating this “fake” temp table.

DROP TABLE IF EXISTS TMP_{from_table_name.split('.')[-1]};

CREATE TABLE TMP_{from_table_name.split(".")[-1]} (
    {", ".join(column_definitions)}
);

As you can see, in doing this we switched the approach for creating the table and manually built the column_definitions. We did this by querying the sys.columns and sys.types tables in MS SQL:

# Query to get column definitions, including identity property
get_columns_query = f"""
    SELECT 
        c.name AS COLUMN_NAME,
        t.name AS DATA_TYPE,
        c.max_length AS COLUMN_LENGTH,
        c.precision AS PRECISION_VALUE,
        c.scale AS SCALE_VALUE,
        d.definition AS COLUMN_DEFAULT,
        COLUMNPROPERTY(c.object_id, c.name, 'IsIdentity') AS IS_IDENTITY
    FROM sys.columns c
    JOIN sys.types t ON c.user_type_id = t.user_type_id
    LEFT JOIN sys.default_constraints d ON c.default_object_id = d.object_id
    WHERE c.object_id = OBJECT_ID('{from_table_name}')
"""

columns = self.connection.execute(get_columns_query).fetchall()
self.logger.debug(f"Fetched columns: {columns}")

# Construct the CREATE TABLE statement
column_definitions = []
for col in columns:
    col_name = col[0]
    col_type = col[1]
    col_length = col[2]
    precision_value = col[3]
    scale_value = col[4]
    col_default = f"DEFAULT {col[5]}" if col[5] else ""
    is_identity = col[6]

    identity_str = "IDENTITY(1,1)" if is_identity else ""

    # Apply length only if it's a varchar/nvarchar type
    if col_type.lower() in ["varchar", "nvarchar"]:
        col_length_str = "(MAX)" if col_length == -1 else f"({col_length})"
    else:
        col_length_str = ""

    column_definitions.append(f"[{col_name}] {col_type}{col_length_str} {identity_str} {col_default}")

Finally, we were able to test our change with a batch of 10,000 records:

2025-01-15 19:01:55,224 Starting 'Load records in database!! Table TMP_contacts; Records count 10000'...
2025-01-15 19:06:35,295 'Load records in database!! Table TMP_contacts; Records count 10000' took 4:40 (mm:ss) to run.

Yikes… 4 minutes 40 sec just to insert 10,000 records was not even close to enough of a performance gain. Worse still, the timeout issues persisted.

Using bcp for the inserts

After that realization, we went back to the drawing board and did some research on what the most performant ways to write to Microsoft SQL Server are. This is when we found Microsoft’s bulk copy program utility (aka bcp) – if you’re like me, that name made you chuckle :)

The concept of bcp is pretty simple, it’s a CLI that enables you to insert CSV files into Microsoft SQL Server super quickly. Plus, it’s designed specifically for loading large amounts of data.

We were a little dubious that bcp could be that much faster, so we did a local test. It did not disappoint – we were able to write 10k rows in just 8 seconds. We had found our solution!

bcp {db} in test-data.csv -S "host" -U "user" -P "pass" -c -t"\\t"  -e "error_log.txt"

Updating the target was a little trickier.

We decided to continue using pyodbc for everything except the inserts. We would use it to create the tables, adding missing columns, ensure schemas, and create the “fake” temp tables. Note that we kept our “fake” temp table solution intact because bcp (similar to fast_executemany) behaved strangely with local temporary tables.

To generate the CSV files for bcp to insert we decided to use pandas – the code was pretty straightforward. We converted our array of insert_records (a list of dictionaries) into a dataframe. Notice that we had to remove new lines and tabs from the input data to avoid parsing errors with bcp . Finally, we serialize the dataframe to a tab delimited CSV with no header.

# Convert the array of records to a dataframe
df = pd.DataFrame(insert_records)
# Remove new lines and tabs to avoid parsing errors with bcp
df = df.replace(r"[\\n\\r\\t]", " ", regex=True)
# Serialize the chunk to CSV with no headers and using tab separators
df.to_csv(f"{table_name}.csv", index=False, header=False, sep="\\t")

Now, for the fun part! We dropped the old INSERT code and replaced it with a call to bcp, passing the fully qualified table name, database credentials, and the path to the CSV file.

# The JOB_ROOT env var here indicates if we're running in hotglue or just locally
bcp = "/opt/mssql-tools/bin/bcp" if os.environ.get("JOB_ROOT") else "bcp"
# Fully qualified table name to insert to
db = f'"[{database}].[{db_schema}].[{table_name}]"'
# Build the bcp command to insert the data
bcp_cmd = f'{bcp} {db} in {table_name}.csv -S "{host}" -U "{user}" -P "{password}" -c -t"\\t"  -e "error_log.txt"'
# Run the command using subprocess
result = subprocess.run(
    bcp_cmd,
    shell=True, capture_output=True, text=True
)
# Log the output
self.logger.info(result.stdout)
if result.stderr:
		# If there are parsing on insert errors we will capture them here
    self.logger.error(result.stderr)

The rest of the code stayed the same (and used pyodbc) – once the insert completes, we simply merge this temp table into the main table + drop the temp table.

With all our changes done, we ran a test in production and saw a huge impact: runtime was down to 1.6 sec to upload the data to temp tables! Our logs below:

2025-01-15 21:04:05,473 /opt/mssql-tools/bin/bcp "[db].[salesforce].[temp_contact]" in temp_contact.csv -S sample.database.windows.net -U admin -c -t"\\t" -e "error_log.txt"
Starting copy...
1000 rows sent to SQL Server. Total sent: 1000
1000 rows sent to SQL Server. Total sent: 2000
1000 rows sent to SQL Server. Total sent: 3000
1000 rows sent to SQL Server. Total sent: 4000
1000 rows sent to SQL Server. Total sent: 5000
1000 rows sent to SQL Server. Total sent: 6000
1000 rows sent to SQL Server. Total sent: 7000
1000 rows sent to SQL Server. Total sent: 8000
1000 rows sent to SQL Server. Total sent: 9000
1000 rows sent to SQL Server. Total sent: 10000
10000 rows copied.
Network packet size (bytes): 4096
Clock Time (ms.) Total : 1644 Average : (6082.7 rows per sec.)
2025-01-15 21:04:07,502 Merging data from temp table to salesforce.contact
2025-01-15 21:04:08,084 Dropping temp table as batch is done salesforce.contact

Conclusion

There were a few small clean up things to do (e.g. adding support for databases on custom ports), but these changes transformed our Microsoft SQL target to a far more performant and reliable connector. Our customer was able to sync millions of CRM records into their Azure SQL Server instance with ease.

If you’re interested in checking out the code in more detail, check out the GitHub repo: https://github.com/hotgluexyz/target-mssql/

Thanks for reading, and stay tuned for more deep dives into how we are building high-performance connectors!

Optimizing Microsoft SQL insert speed in Python: pymssql vs. pyodbc vs. bcp

Connectivity issues with pymssql

Switching to pyodbc

Trying to boost pyodbc performance

Using bcp for the inserts

Conclusion

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