Amazon Redshift
Database Developer Guide

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Split Your Load Data into Multiple Files

The COPY command loads the data in parallel from multiple files, dividing the workload among the nodes in your cluster. When you load all the data from a single large file, Amazon Redshift is forced to perform a serialized load, which is much slower. Split your load data files so that the files are about equal size, between 1 MB and 1 GB after compression. For optimum parallelism, the ideal size is between 1 MB and 125 MB after compression. The number of files should be a multiple of the number of slices in your cluster. For more information about how to split your data into files and examples of using COPY to load data, see Loading Data from Amazon S3.