How to safely unload/copy a table in RedShift? - amazon-redshift

Question

In RedShift, it is convenient to use unload/copy to move data to S3 and load back to redshift, but I feel it is hard to choose the delimiter each time. The right delimiter is relevant to the content of the table! I had to change the delimiter each time I met load errors.
For example, when I use the following command to unload/copy a table:
unload ('select * from tbl_example') to 's3://s3bucket/tbl_example' CREDENTIALS 'aws_access_key_id=xxx;aws_secret_access_key=xxx' delimiter '|' addquotes allowoverwrite;
copy tbl_example2 from 's3://s3bucket/tbl_example' CREDENTIALS 'aws_access_key_id=xxx;aws_secret_access_key=xxx' delimiter '|' removequotes;
I will get load error if the table happens to have a field with its content as "||". Then I have to change the delimiter '|' to another one like ',' and try again, if I'm unlucky, maybe it takes multiple tries to get a success.
I'm wondering if there's a way to unload/copy a redshift table which is irrelevant to the content of the table, which will always succeed no mater what weird strings are stored in the table.

Answer 1

Finally I figured out the right approach, to add escape in both unload and copy command:
unload ('select * from tbl_example') to 's3://s3bucket/tbl_example' CREDENTIALS 'aws_access_key_id=xxx;aws_secret_access_key=xxx' delimiter '|' addquotes escape allowoverwrite;
copy tbl_example2 from 's3://s3bucket/tbl_example' CREDENTIALS 'aws_access_key_id=xxx;aws_secret_access_key=xxx' delimiter '|' removequotes escape;
With escape in unload command, for CHAR and VARCHAR columns in delimited unload files, an escape character (\) is placed before every occurrence of the following characters:
Linefeed: \n
Carriage return: \r
The delimiter character specified for the unloaded data.
The escape character: \
A quote character: " or ' (if both ESCAPE and ADDQUOTES are specified
in the UNLOAD command).
And with escape in copy command, the backslash character () in input data is treated as an escape character. The character that immediately follows the backslash character is loaded into the table as part of the current column value, even if it is a character that normally serves a special purpose. For example, you can use this option to escape the delimiter character, a quote, an embedded newline, or the escape character itself when any of these characters is a legitimate part of a column value.

Answer 2

Try unload like below
unload ('select * from tbl_example') to 's3://s3bucket/tbl_example' CREDENTIALS 'aws_access_key_id=xxx;aws_secret_access_key=xxx' delimiter as ',' addquotes escape
To load it back use as below
copy tbl_example2 from 's3://s3bucket/tbl_example' CREDENTIALS 'aws_access_key_id=xxx;aws_secret_access_key=xxx' delimiter ',' removequotes escape;
This will work irrespective of your data might have , in between.

Answer 3

Since this topic comes up in many places we decided to package up the UNLOAd/extract process into a Docker service. All the code is on Github so you can use it as-is or grab the underlying Python code to create your own version: https://github.com/openbridge/ob_redshift_unload
You can set the delimiter, dates and ad hoc SQL via run-time configuration. This will also export a header row as well, something that is a little more complicated to undertake.
Here are a few of the runtime options:
-t: The table you wish to UNLOAD
-f: The S3 key at which the file will be placed
-s (Optional): The file you wish to read a custom valid SQL WHERE clause from. This will be sanitized then inserted into the UNLOAD command.
-r (Optional): The range column you wish to use to constrain the results. Any type supported by Redshift's BETWEEN function is accepted here (date, integer, etc.)
-r1 (Optional): The desired start range to constrain the result set
-r2 (Optional): The desired end range to constrain the result set
Note: -s and -d are mutually exlusive and cannot be used together. If neither is used, the script will default to not specifying a WHERE clause and output the entire table.
Then you can run it like this to UNLOAD:
docker run -it -v /local/path/to/my/config.json:/config.json openbridge/ob_redshift_unload python /unload.py -t mytable -f s3://dest-bucket/foo/bar/output_file.csv -r datecol -r1 2017-01-01 -r2 2017-06-01
The goal was to enhance the default UNLOAD process and wrap it into something that can help ensure consistency in generating outputs.
Here is a write-up that details the features/capabilities:
https://blog.openbridge.com/how-to-easily-extract-data-from-amazon-redshift-4e55435f7003