Wild guess: this is a left-over from the old days of Sun SPARC processors which could only access doubles on 8-byte aligned memory addresses.
For Intel processors you just paid a performance penality for unaligned accesses but SPARC processors would generate a TRAP for unaligned accesses.
I'd really like to see some performance testing of alignment for modern Intel/ARM CPUs. My guess is that tightly packing would be better as more data would fit into caches and you would have less i/o.
Modern CPUs are very different from back in the 90's when PostgreSQL was designed. Posgresql has traditionally been very slow to stop supporting old OSes that are no longer used.
For Intel processors you just paid a performance penality for unaligned accesses but SPARC processors would generate a TRAP for unaligned accesses.
I'd really like to see some performance testing of alignment for modern Intel/ARM CPUs. My guess is that tightly packing would be better as more data would fit into caches and you would have less i/o.
Modern CPUs are very different from back in the 90's when PostgreSQL was designed. Posgresql has traditionally been very slow to stop supporting old OSes that are no longer used.