Re: Performance of UPDATE SET = FROM vs UPDATE SET = (SELECT ...)

These updates aren't equivalent. It's very important you know this, because you're also inflating your table with a lot
ofextra updated rows. 

Take the first UPDATE:

> UPDATE second SET time1 = orig.time1
> FROM orig
> WHERE second.key1 = orig.key1;

If you wrote this as a SELECT, it would look like this:

SELECT second.time1, orig.time1
  FROM second
  JOIN ORIG ON (second.key1 = orig.key1)

Since second is a many to one subset of orig, you now have several simultaneous updates. Your second UPDATE:

> UPDATE second SET time1 = (SELECT orig.time1 FROM orig,second
> WHERE orig.key1 = second.key1 LIMIT 1);

Is equivalent to this SELECT:

SELECT second.time1,
       (SELECT orig.time1 FROM orig,second
         WHERE orig.key1 = second.key1 LIMIT 1)
  FROM second;

Meaning you'd only get as many updates as there are rows in second. The difference is your LIMIT 1. However, since
you'renot using an ORDER BY clause, the actual value you get for time1 will be indeterminate. Something like this would
removethe row inflation and fix the random time1 behavior, but I'm not sure it was your intent: 

UPDATE second
   SET time1 = orig.time1
  FROM (SELECT DISTINCT ON (key1) key1, time1
          FROM orig
         ORDER BY key1, time1 DESC) sub
 WHERE second.key1 = sub.key1;


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Why does the UPDATE SET = FROM choose a more poorly performing plan than
the UPDATE SET = (SELECT ...)?  It seems to me that it is the same join.

I'm using 9.3.5.

CREATE TABLE orig
(
        key1    VARCHAR(11) PRIMARY KEY,
        time1   TIME
);

INSERT INTO orig (key1, time1)
SELECT
        a::TEXT,
        (((random()*100)::INT % 24)::TEXT || ':' ||
        ((random()*100)::INT % 60)::TEXT)::TIME
FROM generate_series(80000000000, 80002000000) a;

CREATE INDEX odx ON orig(key1);

CREATE TABLE second (LIKE orig);

INSERT INTO second (key1)
        SELECT (80000000000+(((random()*1000000)::INT) % 1000000))::TEXT
        FROM generate_series(1,400000);

EXPLAIN ANALYZE
UPDATE second SET time1 = orig.time1
FROM orig
WHERE second.key1 = orig.key1;

                                                             QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------
 Update on second  (cost=69461.02..106082.02 rows=400000 width=32) (actual time=16033.023..16033.023 rows=0 loops=1)
   ->  Hash Join  (cost=69461.02..106082.02 rows=400000 width=32) (actual time=7698.445..12992.039 rows=400000 loops=1)
         Hash Cond: ((second.key1)::text = (orig.key1)::text)
         ->  Seq Scan on second  (cost=0.00..12627.00 rows=400000 width=18) (actual time=49.820..791.397 rows=400000
loops=1)
         ->  Hash  (cost=31765.01..31765.01 rows=2000001 width=26) (actual time=7648.540..7648.540 rows=2000001
loops=1)
               Buckets: 4096  Batches: 128  Memory Usage: 717kB
               ->  Seq Scan on orig  (cost=0.00..31765.01 rows=2000001 width=26) (actual time=0.014..3655.844
rows=2000001loops=1) 
 Total runtime: 16033.193 ms
(8 rows)

UPDATE second SET time1 = NULL;

EXPLAIN ANALYZE
UPDATE second SET time1 = (SELECT orig.time1 FROM orig,second
                        WHERE orig.key1 = second.key1 LIMIT 1);


                                                               QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------------
 Update on second  (cost=3.60..19078.19 rows=1279959 width=18) (actual time=4642.453..4642.453 rows=0 loops=1)
   InitPlan 1 (returns $1)
     ->  Limit  (cost=0.43..3.60 rows=1 width=8) (actual time=2.611..2.613 rows=1 loops=1)
           ->  Nested Loop  (cost=0.43..4056331.83 rows=1279959 width=8) (actual time=2.606..2.606 rows=1 loops=1)
                 ->  Seq Scan on second second_1  (cost=0.00..19074.59 rows=1279959 width=12) (actual time=2.487..2.487
rows=1loops=1) 
                 ->  Index Scan using odx on orig  (cost=0.43..3.14 rows=1 width=20) (actual time=0.098..0.098 rows=1
loops=1)
                       Index Cond: ((key1)::text = (second_1.key1)::text)
   ->  Seq Scan on second  (cost=0.00..19074.59 rows=1279959 width=18) (actual time=6.420..817.739 rows=400000 loops=1)
 Total runtime: 4642.561 ms
(9 rows)




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