Index not being used for UPDATE?

[Phoenix Kiula <phoenix.kiula@xxxxxxxxx>]

Hi.

Just want to check why, in an UPDATE sql, the JOIN condition is not making use of the index? 

In both tables being joined, the column in question is in fact the primary key! 

Table structure and query below. All I want is to take values from a smaller "accesscount" table and update from it the values in the TABLE1 table, which is a larger table.

The query plan shows sequential scan of both the tables. Why is this and how can I work around it? 

Thanks! 

  

Table "public.TABLE1"

   Column    |            Type             |            Modifiers            
--------------+-----------------------------+---------------------------------
alias        | character varying(35)       | not null
som          | text                        | not null
user_id      | character varying(30)       | not null
modify_date  | timestamp without time zone | default now()
volatility   | character varying(32)       |
acount       | integer                     |
Indexes:
   "idx_TABLE1_pkey" PRIMARY KEY, btree (alias)
   "idx_TABLE1_userid" btree (user_id) CLUSTER




Table "public.accesscount"

   Column    |         Type          | Modifiers
--------------+-----------------------+-----------
alias        | character varying(35) | not null
acount       | integer               |
Indexes:
   "idx_9" PRIMARY KEY, btree (alias)




=# explain
update TABLE1
   set acount = v.acount
from accesscount v
where TABLE1.alias = v.alias
;

                                       QUERY PLAN                                        
------------------------------------------------------------------------------------------
Update  (cost=22985.69..1088981.66 rows=613453 width=173)
  ->  Hash Join  (cost=22985.69..1088981.66 rows=613453 width=173)
        Hash Cond: ((TABLE1.alias)::text = (v.alias)::text)
        ->  Seq Scan on TABLE1  (cost=0.00..410625.10 rows=12029410 width=159)
        ->  Hash  (cost=11722.53..11722.53 rows=613453 width=21)
              ->  Seq Scan on accesscount v  (cost=0.00..11722.53 rows=613453 width=21)
(6 rows)

Time: 0.848 ms