title: Selecting from JSONB field slow
body: <p>I have a relatively small table (~50k rows). When I select all records, it takes a ~40s. The table has 3 JSONB columns. When I select every column except for the JSONBs, the query takes ~700ms.</p>
  
  <p>If I add in just one of the JSONB fields, the query time jumps to nearly 10s.</p>
  
  <p>I'm never using a where clause referencing something inside the JSONB, just selecting *. Even so, I tried adding GIN indexes because I saw them frequently mentioned as a performance booster for JSONB.</p>
  
  <p>I've ran a full vacuum.</p>
  
  <p>Postgres version 9.6</p>
  
  <pre><code> explain (analyze, buffers) select * from message;
     Seq Scan on message  (cost=0.00..5541.69 rows=52969 width=834) (actual 
     time=1.736..116.183 rows=52969 loops=1)
       Buffers: shared hit=64 read=4948
     Planning time: 0.151 ms
     Execution time: 133.555 ms
  </code></pre>