Function
| Arg | Description | Type |
|---|---|---|
| message | Message to log. | string (required) |
| dedup | Suppress same message in this many seconds (default 60 sec). Use -1 to disable dedup. | int64 |
| args | An array of elements to apply into the format string. | Any |
| level | Level to log at (DEFAULT, WARN, ERROR, INFO, DEBUG). | string |
Log a message to the query log stream. Always returns TRUE.
The message parameter represents a format string
that will be expanded using the args parameter list if needed.
Since log() always returns TRUE it is easy to use in a WHERE
clause as a form of debugging. It is basically equivalent to the
print statement of other languages.
SELECT * FROM glob(...)
WHERE log(message="Value of OSPath is %v", args=OSPath)
Log messages will be deduped according to the dedup
parameter - each distinct format string will not be emitted more
frequently than the dedup parameter (by default 60 seconds).
This makes it safe to use log() frequently without flooding
the logs stream.
SELECT * FROM range(end=_value)
WHERE log(message="Value is %v", args=_value)
Will only emit a single message due to the format string being deduped.
This property makes it useful to add progress logging to long running artifacts. The logs will be emitted every minute.
SELECT * FROM glob(...)
WHERE log(message="Processing file %v", args=OSPath)
In this more complex example the query will produce 10 rows, at a rate of 1 row every 5 seconds. However the log messages will be limited to 1 every 15 seconds.
SELECT count() AS Count, String AS EventTime FROM clock(period=5)
WHERE log(message="Logging #%v at %v", args=[Count, EventTime], level="INFO", dedup=15)
LIMIT 10
Thus the log message will be emitted for the 1st, 4th, 7th, and 10th rows. To observe the deduplication behaviour in real time you can run this query in a notebook cell and tweak the arguments to understand their impacts.