Re: jsonb format is pessimal for toast compression

Curious idea: we could swap JEntry array and values: values in the
begining of type will be catched by pg_lzcompress. But we will need to
know offset of JEntry array, so header will grow up till 8 bytes
(actually, it will be a varlena header!)

May be I wasn't clear:jsonb type will start from string collection instead of JEntry array, JEntry array will be placed at the end of object/array. so, pg_lzcompress will find repeatable 4-byte pieces in first 1024 bytes of jsonb.