It's the same as bytea but with 8-byte alignment. My understanding of why it was added (this was before my time with MADlib) is that most of the models used Eigen which worked faster with SSE2. The 8-byte alignment helped in running the SSE2 instructions. 

On Tue, Jul 19, 2016 at 2:59 PM, Jim Nasby <Jim.Nasby@bluetreble.com> wrote:
On 7/19/16 3:25 PM, Rahul Iyer wrote:
The error message indicated that the madlib.bytea8 composite type was
missing or invalid.

Out of curiosity, why is that type needed?
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