Is it possible to use wildcards for an S3 key (filename) to allow Talend to. The other nice side effect of PARALLEL OFF is that it will respect your ORDER BY clause if you have one and generate the files in an order that keeps all the records ordered, even across multiple output files.Īddendum: There seems to be some folkloric knowledge around using LIMIT 2147483647 to force the leader node to do all the processing and generate a single output file, but this doesn't seem to be actually documented anywhere in the Redshift documentation and as such, relying on it seems like a bad idea since it could change at any time. Question: I am trying to copy data from S3 bucket into Redshift using copy. The following example uses the myextunloadstage stage to unload all the. It's also not clear to me if the GZIP option affects the output file size spillover limit or not (it's unclear if 6.2GB is the pre-GZIP size limit or the post-GZIP size limit).įor me, the UNLOAD command that ending up generating a single CSV file in most cases was: UNLOADĬREDENTIALS 'aws_access_key_id= aws_secret_access_key=' If you already have a Amazon Web Services (AWS) account and use S3 buckets. Unfortunately neither of these techniques are perfect since rows can be of variable size. So you need to make your query have a more restrictive WHERE clause or use a LIMIT clause to keep the number of records down. The raw size of the data must be less than 6.2GB if you want a single file. PARALLEL OFF will write files serially to S3 instead of in parallel and will only spill over to using multiple files if you exceed the size limit. Parallel is "ON" by default and will generally write to multiple files unless you have a tiny cluster (The number of output files with "PARALLEL ON" set is proportional to the number of slices in your cluster). So if you want to try to guarantee that you get a single output file from UNLOAD, here's what you should try: Now let us consider one example where we have a scenario that we have to unload a table present in redshift to a CSV file in the S3 bucket. The limit on the size of an output file generated by the Redshift UNLOAD command is 6.2GB. Given below is the example of RedShift UNLOAD: The use of the unload command, and its purpose can vary depending on the scenario where they have to be used. Per the documentation ( ), the main factor in limiting the number of files you generate is the actual raw size in bytes of your export ( NOT the number of rows). That being said, for most cases, you can generally limit your query in such a way that you'll end up with a single file. If your question is, "Can I absolutely 100% guarantee that Redshift will ALWAYS unload to a SINGLE file in S3?", the answer is simply NO. Therefore, you can, and should, select the best File Type to use when unloading data. Note that the tRedshiftUnload component supports unloading to multiple file types. You run a specific query on Amazon Redshift, then unloads the query results into one or more files on S3. This is an old question at this point, but I feel like all the existing answers are slightly misleading. It allows you to unload data on Amazon Redshift to S3.
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