Troubleshooting

Common issues

You may be experiencing one of the issues below.

Rows accepted over QWP but not visible to queries

flush(wait=True) (and QuestDB.dataframe) confirm the server accepted the rows; query visibility follows once the WAL is applied, which is asynchronous. Poll for the condition you need with a bounded timeout instead of asserting immediately after the flush.

Where did my QWP server rejection go?

Over QWP/WebSocket a server rejection (schema mismatch, parse error, …) can arrive after flush() already returned. Every rejection is pushed to the error_handler passed to questdb.connect(); without one it is logged through the questdb Python logger — ERROR for terminal rejections, WARNING for retriable ones (the queue replays those). Make sure Python logging is configured to show the questdb logger, or register a handler. Terminal rejections additionally raise QuestDBServerRejectionError from the next call on the affected sender.

Production-optimized QuestDB configuration

If you can’t initially see your data through a select SQL query straight away, this is normal: by default the database will only commit data it receives though the line protocol periodically to maximize throughput.

For dev/testing you may want to tune the following database configuration parameters as so:

# server.conf
cairo.max.uncommitted.rows=1
line.tcp.maintenance.job.interval=100

The default QuestDB configuration is more applicable for a production environment.

For these and more configuration parameters refer to database configuration documentation.

Infrequent Flushing

You may not see data appear in a timely manner because you’re not calling flush often enough.

You might be having issues with the Sender’s auto-flush feature.

Errors during flushing

Decimal Column Errors

If you’re trying to ingest decimal data and encountering errors, check the following:

Pre-create table: Unlike other column types, DECIMAL columns cannot be auto-created. You must create the table with DECIMAL(precision, scale) columns before sending data:

CREATE TABLE my_table (
    symbol SYMBOL,
    price DECIMAL(18, 6),
    timestamp TIMESTAMP_NS
) TIMESTAMP(timestamp) PARTITION BY DAY;

Protocol version mismatch: Decimal support requires protocol version 3, which is only available on QuestDB server 9.2.0 or later.

• For HTTP/HTTPS: Protocol version 3 is auto-negotiated. Ensure your server is version 9.2.0 or later.

• For TCP/TCPS: You must explicitly configure protocol_version=3 in your configuration string:

  tcp::addr=localhost:9009;protocol_version=3;
  

Precision/scale mismatch: Ensure the precision and scale of your Python decimal.Decimal or PyArrow decimal values match the table definition. For example, if the table has DECIMAL(12, 6), values with more than 6 decimal places or more than 12 total digits will cause errors.

For more details on decimal types, see the QuestDB DECIMAL documentation.

ILP/TCP Server disconnects

If you’re using TCP instead of HTTP, you may see a server disconnect after flushing.

If the server receives invalid data over ILP/TCP it will drop the connection.

The ILP/TCP protocol does not send errors back to the client. Instead, by design, it will disconnect a client if it encounters any insertion errors. This is to avoid errors going unnoticed.

As an example, if a client were to insert a STRING value into a BOOLEAN column, the QuestDB server would disconnect the client.

To determine the root cause of a disconnect, inspect the server logs.

Note

For a better developer experience consider using HTTP instead of TCP.

Logging outgoing messages

To understand what data was sent to the server, you may log outgoing messages from Python.

Over the text-based ILP transports, bytes(sender) exposes the pending payload (QWP transports encode at flush time, so it is empty there). Here’s an example if you append rows to the Sender object:

import logging
import textwrap

with Sender.from_conf(...) as sender:
    # sender.row(...)
    # sender.row(...)
    # ...
    pending = bytes(sender).decode('utf-8', errors='replace')
    logging.info('About to flush:\n%s', textwrap.indent(pending, '    '))
    sender.flush()

Alternatively, if you’re constructing buffers explicitly through the legacy questdb.ingress shim:

import logging
import textwrap

buffer = sender.new_buffer()
# buffer.row(...)
# buffer.row(...)
# ...
pending = bytes(buffer).decode('utf-8', errors='replace')
logging.info('About to flush:\n%s', textwrap.indent(pending, '    '))
sender.flush(buffer)

Note that to handle out-of-order messages efficiently, the QuestDB server will delay applying changes it receives over ILP after a configurable commit lag.

Due to this commit lag, the line that caused the error may not be the last line.

Asking for help

The best way to get help is through our Community Forum.