Re: Number of characters in column preventing index usage

["David G. Johnston" <david.g.johnston@xxxxxxxxx>]

On Fri, Feb 17, 2017 at 3:19 PM, Hustler DBA <hustlerdba@xxxxxxxxx> wrote:

my_db=# create index tab_idx1 on tab(ID);                                                               

CREATE INDEX

my_db=# explain (analyze, buffers) select count(*) from tab where ID = '01625cfa-2bf8-45cf' ;                  

                                                              QUERY PLAN                                                               

---------------------------------------------------------------------------------------------------------------------------------------

 Aggregate  (cost=8.29..8.30 rows=1 width=0) (actual time=0.048..0.048 rows=1 loops=1)

   Buffers: shared read=2

   ->  Index Only Scan using tab_idx1 on tab  (cost=0.27..8.29 rows=1 width=0) (actual time=0.043..0.043 rows=0 loops=1)

         Index Cond: (ID = '01625cfa-2bf8-45cf'::text)

 

   ->  Seq Scan on tab  (cost=0.00..14.79 rows=5 width=0) (actual time=0.031..0.108 rows=5 loops=1)

         Filter: ((ID)::text = '01625cfa-2bf8-45cf-bf4c-aa5f3c6fa8ea'::text)

         Rows Removed by Filter: 218

         Buffers: shared hit=12

 Planning time: 0.122 ms

 Execution time: 0.180 ms

(8 rows)

​IIRC the only reason the first query cares to use the index is because it can perform an Index Only Scan and thus avoid touching the heap at all.  If it cannot avoid touching the heap the planner is going to just use a sequential scan to retrieve the records directly from the heap and save the index lookup step.

David J.