Question

I have two tables

junk=# select * from t;                                                                                                                                
   name   | intval 
----------+--------
 bar2  |      2
 bar3  |      3
 bar4  |      4
(3 rows)

and

junk=# select * from temp;                                                                                                                             
 id |    name    | intval 
----+------------+--------
  1 | foo   |      0
  2 | foo2  |      2
  3 | foo3  |      3
  4 | foo4  |      4
  5 | foo5  |      5
(5 rows)

Now, I want to use the values from table t to update the values in table temp. Basically, I want to replace the name column in second, third and fourth values in temp by bar2, bar3 and bar4.

I created the table t using the COPY statement. I am doing batch updates and I am trying to optimize that.

So, I get this error. I think this is pretty basic one.

junk=# UPDATE temp FROM t SET name=t.name FROM t WHERE intval=t.intval;
ERROR:  syntax error at or near "FROM"
LINE 1: UPDATE temp FROM t SET name=t.name FROM t WHERE intval=t.int...
                    ^
junk=# 

Fow now, this works.

UPDATE test SET name=t.name FROM t WHERE test.intval=t.intval

Answer 1

Get rid of your first FROM t clause.

FROM must come after SET, not before and it can only affect the WHERE clause. SET must be done with subqueries.

your completed code is:

UPDATE temp SET name=(SELECT t.name FROM t WHERE temp.intval = t.inval);

PostgreSQL has some ways to optimize this so it’s not like you are just doing a huge nested loop join (and looking up one row over and over from the heap based on the join criteria).

Edit: Adding plan to show we are not, in fact, running through a sequential scan of the second table for each row on the first one.

Here is an example that updates 172 rows in one table using a group-by from another:

mtech_test=# explain analyze
update ap
set amount = (select sum(amount) from acc_trans ac where ac.trans_id = ap.id) + 1;
                                                                 QUERY PLAN 

--------------------------------------------------------------------------------
---------------------------------------------------------------------
Update on ap  (cost=0.00..3857.06 rows=229 width=231) (actual time=39.074..39.0
   74 rows=0 loops=1)
 ->  Seq Scan on ap  (cost=0.00..3857.06 rows=229 width=231) (actual time=0.050..28.444 rows=172 loops=1)
     SubPlan 1
       ->  Aggregate  (cost=16.80..16.81 rows=1 width=5) (actual time=0.109..0.110 rows=1 loops=172)
             ->  Bitmap Heap Scan on acc_trans ac  (cost=4.28..16.79 rows=4 width=5) (actual time=0.075..0.102 rows=4 loops=172)
                   Recheck Cond: (trans_id = ap.id)
                   ->  Bitmap Index Scan on acc_trans_trans_id_key  (cost=0.00..4.28 rows=4 width=0) (actual time=0.006..0.006 rows=4 loops=172)
                         Index Cond: (trans_id = ap.id)
Trigger for constraint ap_entity_id_fkey: time=69.532 calls=172
Trigger ap_audit_trail: time=391.722 calls=172
Trigger ap_track_global_sequence: time=1.954 calls=172
Trigger check_department: time=111.301 calls=172
Total runtime: 612.001 ms
(13 rows)

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