If there are too few spinning disks per node to support the seek rate (either in an individual query plan, or when approaching optimal concurrency for full CPU utilization), SSD might help speed up queries. To make more cost-effective use of SSDs, you may consider using them only for frequently accessed columns or for temporary data such as spill-to-disk operations like large sorts, JOINs, and GROUP BYs. See Documentation for details.
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