On 4/27/2011 11:15 AM, Phoenix Kiula wrote:
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
Looks to me like it loaded the entire accesscount table into an in memory hash, then it scanned table1 to update each row. Because accessCount is small, it was faster to read all of it at once. If the table grows, at some point (and with the help of work_mem?), I assume PG will switch to looking up rows, which is gonna be slower.
-Andy -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
