Why is this SQL statement so fast?

Question

The table in question has 3.8M records. The data column is indexed on a different field: "idx2_table_on_data_id" btree ((data ->> 'id'::text)). I assumed the sequential scan would be very slow but it is completing in just over 1 second. data->'array' does not exist in many of the records, fyi. Why is this running so quickly? Postgres v10

db=> explain analyze select * from table where jsonb_array_length(data->'array') != 0;
                                                    QUERY PLAN                                                    
------------------------------------------------------------------------------------------------------------------
 Seq Scan on table (cost=0.00..264605.21 rows=3797785 width=681) (actual time=0.090..1189.997 rows=1762 loops=1)
   Filter: (jsonb_array_length((data -> 'array'::text)) <> 0)
   Rows Removed by Filter: 3818154
 Planning time: 0.561 ms
 Execution time: 1190.492 ms
(5 rows)

Answer 1

We could tell for sure if you had run EXPLAIN (ANALYZE, BUFFERS), but odds are that most of the data were cached in RAM.

Also jsonb_array_length(data->'array') is not terribly expensive if the JSON is short.