Merge pull request #198 from halilduygulu/redshift

Added some Redshift tips.

README.md

@@ -1286,6 +1286,8 @@ Redshift
 -	❗ Never resize a live cluster. The resize operation takes hours depending on the dataset size. In rare cases, the operation may also get stuck and you'll end up having a non-functional cluster. The safer approach is to create a new cluster from a snapshot, resize the new cluster and shut down the old one.
 -	Redshift has reserved keywords which are not present in Postgres (see full list [here](https://docs.aws.amazon.com/redshift/latest/dg/r_pg_keywords.html)). Watch out for DELTA ([Delta Encodings](https://docs.aws.amazon.com/redshift/latest/dg/c_Delta_encoding.html)).
 -	Redshift does not support many Postgres functions, most notably several date/time-related and aggregation functions. See the [full list here](https://docs.aws.amazon.com/redshift/latest/dg/c_unsupported-postgresql-functions.html).
+-	🔹 [Choosing a sort key](http://docs.aws.amazon.com/redshift/latest/dg/t_Sorting_data.html) is very important since you can not change a table’s sort key after it is created. If you need to change the sort or distribution key of a table, you need to create a new table with the new key and move your data into it with a query like “insert into new_table select * from old_table”. 
+-	❗🚪 When moving data with a query that looks like “insert into x select from y”, you need to have twice as much disk space available as table “y” takes up on the cluster’s disks. Redshift first copies the data to disk and then to the new table. [Here](https://www.periscopedata.com/blog/changing-dist-and-sort-keys-in-redshift.html) is a good article on how to this for big tables.
 
 EMR
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