Thread: UNION versus SUB SELECT

I have found this:

SELECT c.* 

FROM contacts c

WHERE c.id IN ( SELECT p.contact_id FROM phone p WHERE p.addr = ? )

OR c.id IN (SELECT e.contact_id FROM email e WHERE e.addr = ? );

To have a worse plan than:

SELECT * FROM contacts where id IN (

( SELECT c.id FROM contacts c 

JOIN phone p ON c.id = p.contact_id AND p.addr = ? 

UNION

SELECT c.id FROM contacts c 

JOIN email e ON c.id = e.contact_id AND e.addr = ? );

Maybe this is no surprise. But after discovering this my question is this, is there another option I dont' know about that is logically the same that can perform even better than the UNION?

Hi Robert, could you try with "exists" ?

SELECT c.* 

FROM contacts c

WHERE  exists  ( SELECT  1 FROM phone p WHERE p.addr =? and  p.contact_id=c.id )

OR exists (SELECT  1 FROM email e WHERE e.addr = ? and  e.contact_id=c.id );

2013/11/21 Robert DiFalco <robert.difalco@gmail.com>

I have found this:

SELECT c.* 

FROM contacts c

WHERE c.id IN ( SELECT p.contact_id FROM phone p WHERE p.addr = ? )

OR c.id IN (SELECT e.contact_id FROM email e WHERE e.addr = ? );

To have a worse plan than:

SELECT * FROM contacts where id IN (

( SELECT c.id FROM contacts c 

JOIN phone p ON c.id = p.contact_id AND p.addr = ? 

UNION

SELECT c.id FROM contacts c 

JOIN email e ON c.id = e.contact_id AND e.addr = ? );

Maybe this is no surprise. But after discovering this my question is this, is there another option I dont' know about that is logically the same that can perform even better than the UNION?

UNION and subselect both performed better than EXISTS for this particular case.

On Thu, Nov 21, 2013 at 12:31 PM, desmodemone <desmodemone@gmail.com> wrote:

Hi Robert, could you try with "exists" ?

SELECT c.* 

FROM contacts c

WHERE  exists  ( SELECT  1 FROM phone p WHERE p.addr =? and  p.contact_id=c.id )

OR exists (SELECT  1 FROM email e WHERE e.addr = ? and  e.contact_id=c.id );

2013/11/21 Robert DiFalco <robert.difalco@gmail.com>

I have found this:

SELECT c.* 

FROM contacts c

WHERE c.id IN ( SELECT p.contact_id FROM phone p WHERE p.addr = ? )

OR c.id IN (SELECT e.contact_id FROM email e WHERE e.addr = ? );

To have a worse plan than:

SELECT * FROM contacts where id IN (

( SELECT c.id FROM contacts c 

JOIN phone p ON c.id = p.contact_id AND p.addr = ? 

UNION

SELECT c.id FROM contacts c 

JOIN email e ON c.id = e.contact_id AND e.addr = ? );

Maybe this is no surprise. But after discovering this my question is this, is there another option I dont' know about that is logically the same that can perform even better than the UNION?

Could you please attache the plan with explain buffers verbose?

thank you

2013/11/21 Robert DiFalco <robert.difalco@gmail.com>

UNION and subselect both performed better than EXISTS for this particular case.

On Thu, Nov 21, 2013 at 12:31 PM, desmodemone <desmodemone@gmail.com> wrote:

Hi Robert, could you try with "exists" ?

SELECT c.* 

FROM contacts c

WHERE  exists  ( SELECT  1 FROM phone p WHERE p.addr =? and  p.contact_id=c.id )

OR exists (SELECT  1 FROM email e WHERE e.addr = ? and  e.contact_id=c.id );

2013/11/21 Robert DiFalco <robert.difalco@gmail.com>

I have found this:

SELECT c.* 

FROM contacts c

WHERE c.id IN ( SELECT p.contact_id FROM phone p WHERE p.addr = ? )

OR c.id IN (SELECT e.contact_id FROM email e WHERE e.addr = ? );

To have a worse plan than:

SELECT * FROM contacts where id IN (

( SELECT c.id FROM contacts c 

JOIN phone p ON c.id = p.contact_id AND p.addr = ? 

UNION

SELECT c.id FROM contacts c 

JOIN email e ON c.id = e.contact_id AND e.addr = ? );

Maybe this is no surprise. But after discovering this my question is this, is there another option I dont' know about that is logically the same that can perform even better than the UNION?

On 2013-11-21 16:12, Robert DiFalco wrote:

Sorry I couldn't get buffers to work but here is the explain analyze verbose:

dft1fjfv106r48=> explain analyze verbose select c.*                                                                                                                    from contacts c                                                                                                                                                        where c.id IN (                                                                                                                                                            select p.contact_id                                                                                                                                                    from phone_numbers p                                                                                                                                                   where (p.national = 5038904993 and p.e164 = '+15038904993'))                                                                                                       or c.id IN (                                                                                                                                                               select e.contact_id                                                                                                                                                    from email_addresses e                                                                                                                                                 where e.email = 'robert.xxxxx@gmail.com')                                                                                                                          ;

                                                                     QUERY PLAN                                                                     

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

 Seq Scan on public.contacts c  (cost=8.12..75.73 rows=1988 width=95) (actual time=0.410..0.410 rows=0 loops=1)

   Output: c.id, c.owner_id, c.user_id, c.device_id, c.last_call, c.record_id, c.dtype, c.blocked, c.details_hash, c.fname, c.lname, c.fb_id

   Filter: ((hashed SubPlan 1) OR (hashed SubPlan 2))

   Rows Removed by Filter: 2849

   SubPlan 1

     ->  Index Scan using idx_phone_address on public.phone_numbers p  (cost=0.06..4.06 rows=1 width=8) (actual time=0.015..0.015 rows=0 loops=1)

           Output: p.contact_id

           Index Cond: ((p."national" = 5038904993::bigint) AND ((p.e164)::text = '+15038904993'::text))

   SubPlan 2

     ->  Index Scan using idx_email_address on public.email_addresses e  (cost=0.06..4.06 rows=1 width=8) (actual time=0.018..0.018 rows=0 loops=1)

           Output: e.contact_id

           Index Cond: ((e.email)::text = 'robert.xxxxx@gmail.com'::text)

 Total runtime: 0.489 ms

(13 rows)

dft1fjfv106r48=> explain analyze verbose select c.*                                                                                                                    from contacts c                                                                                                                                                        where exists(                                                                                                                                                              select 1                                                                                                                                                               from phone_numbers p                                                                                                                                                   where (p.national = 5038904993 and p.e164 = '+15038904993') and p.contact_id = c.id)                                                                               or EXISTS(                                                                                                                                                                 select 1                                                                                                                                                               from email_addresses e                                                                                                                                                 where e.email = 'robert.xxxxx@gmail.com' and e.contact_id = c.id)                                                                                                  ;

                                                                      QUERY PLAN                                                                      

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

 Seq Scan on public.contacts c  (cost=0.00..21596.38 rows=1988 width=95) (actual time=0.479..0.479 rows=0 loops=1)

   Output: c.id, c.owner_id, c.user_id, c.device_id, c.last_call, c.record_id, c.dtype, c.blocked, c.details_hash, c.fname, c.lname, c.fb_id

   Filter: ((alternatives: SubPlan 1 or hashed SubPlan 2) OR (alternatives: SubPlan 3 or hashed SubPlan 4))

   Rows Removed by Filter: 2849

   SubPlan 1

     ->  Index Scan using idx_phone_address on public.phone_numbers p  (cost=0.06..4.06 rows=1 width=0) (never executed)

           Index Cond: ((p."national" = 5038904993::bigint) AND ((p.e164)::text = '+15038904993'::text))

           Filter: (p.contact_id = c.id)

   SubPlan 2

     ->  Index Scan using idx_phone_address on public.phone_numbers p_1  (cost=0.06..4.06 rows=1 width=8) (actual time=0.010..0.010 rows=0 loops=1)

           Output: p_1.contact_id

           Index Cond: ((p_1."national" = 5038904993::bigint) AND ((p_1.e164)::text = '+15038904993'::text))

   SubPlan 3

     ->  Index Scan using idx_email_address on public.email_addresses e  (cost=0.06..4.06 rows=1 width=0) (never executed)

           Index Cond: ((e.email)::text = 'robert.xxxxx@gmail.com'::text)

           Filter: (e.contact_id = c.id)

   SubPlan 4

     ->  Index Scan using idx_email_address on public.email_addresses e_1  (cost=0.06..4.06 rows=1 width=8) (actual time=0.016..0.016 rows=0 loops=1)

           Output: e_1.contact_id

           Index Cond: ((e_1.email)::text = 'robert.xxxxx@gmail.com'::text)

 Total runtime: 0.559 ms

(21 rows)

dft1fjfv106r48=> explain analyze verbose select * from contacts where id IN (                                                                                          (select c.id                                                                                                                                                           from contacts c                                                                                                                                                        join phone_numbers p on c.id = p.contact_id and p.national = 5038904993 and p.e164 = '+15038904993')                                                                   union (select c.id                                                                                                                                                     from contacts c                                                                                                                                                        join email_addresses e on c.id = e.contact_id and e.email = 'robert.xxxxx@gmail.com'));                                                                                                                                                                                              QUERY PLAN                                                                                                               ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

 Nested Loop  (cost=16.31..24.39 rows=2 width=95) (actual time=0.060..0.060 rows=0 loops=1)

   Output: contacts.id, contacts.owner_id, contacts.user_id, contacts.device_id, contacts.last_call, contacts.record_id, contacts.dtype, contacts.blocked, contacts.details_hash, contacts.fname, contacts.lname, contacts.fb_id

   ->  Unique  (cost=16.26..16.26 rows=2 width=8) (actual time=0.057..0.057 rows=0 loops=1)

         Output: c.id

         ->  Sort  (cost=16.26..16.26 rows=2 width=8) (actual time=0.055..0.055 rows=0 loops=1)

               Output: c.id

               Sort Key: c.id

               Sort Method: quicksort  Memory: 25kB

               ->  Append  (cost=0.11..16.25 rows=2 width=8) (actual time=0.034..0.034 rows=0 loops=1)

                     ->  Nested Loop  (cost=0.11..8.12 rows=1 width=8) (actual time=0.013..0.013 rows=0 loops=1)

                           Output: c.id

                           ->  Index Scan using idx_phone_address on public.phone_numbers p  (cost=0.06..4.06 rows=1 width=8) (actual time=0.011..0.011 rows=0 loops=1)

                                 Output: p.id, p.contact_id, p."national", p.e164, p.raw_number

                                 Index Cond: ((p."national" = 5038904993::bigint) AND ((p.e164)::text = '+15038904993'::text))

                           ->  Index Only Scan using idx_contacts_pkey_owner on public.contacts c  (cost=0.06..4.06 rows=1 width=8) (never executed)

                                 Output: c.id, c.owner_id, c.user_id

                                 Index Cond: (c.id = p.contact_id)

                                 Heap Fetches: 0

                     ->  Nested Loop  (cost=0.11..8.12 rows=1 width=8) (actual time=0.018..0.018 rows=0 loops=1)

                           Output: c_1.id

                           ->  Index Scan using idx_email_address on public.email_addresses e  (cost=0.06..4.06 rows=1 width=8) (actual time=0.016..0.016 rows=0 loops=1)

                                 Output: e.id, e.contact_id, e.email

                                 Index Cond: ((e.email)::text = 'robert.xxxxx@gmail.com'::text)

                           ->  Index Only Scan using idx_contacts_pkey_owner on public.contacts c_1  (cost=0.06..4.06 rows=1 width=8) (never executed)

                                 Output: c_1.id, c_1.owner_id, c_1.user_id

                                 Index Cond: (c_1.id = e.contact_id)

                                 Heap Fetches: 0

   ->  Index Scan using idx_contacts_pkey_owner on public.contacts  (cost=0.06..4.06 rows=1 width=95) (never executed)

         Output: contacts.id, contacts.owner_id, contacts.user_id, contacts.device_id, contacts.last_call, contacts.record_id, contacts.dtype, contacts.blocked, contacts.details_hash, contacts.fname, contacts.lname, contacts.fb_id

         Index Cond: (contacts.id = c.id)

 Total runtime: 0.332 ms

(31 rows)

The buffers option is 9.0+ and is used like "explain (analyze, verbose, buffers) select 1".
To your original question, the union output there runs slightly faster than the "in" approach, although this may not be a good example - your inputs don't return any data, so this might not be realistic - and those numbers are so low that the difference might just be noise.
Are you tuning a <0.5ms-runtime query? Or is this just curiosity? FWIW I tend to write queries like this using an exists check first, then if that's still not good enough (all things like proper indexing taken in to account) I'll try an in check, then finally a union if that's still not good enough.

On Thu, Nov 21, 2013 at 2:04 PM, Robert DiFalco <robert.difalco@gmail.com> wrote:

Hmmmm...I'm not sure why the buffers option didn't work for me, maybe the heroku psql is out of date.

Did you enclose it in brackets? Eg. "EXPLAIN (ANALYZE, BUFFERS) SELECT ..."

On Thu, Nov 21, 2013 at 2:31 PM, desmodemone <desmodemone@gmail.com> wrote:
> Hi Robert, could you try with "exists" ?
>
> SELECT c.*
> FROM contacts c
> WHERE  exists  ( SELECT  1 FROM phone p WHERE p.addr =? and
> p.contact_id=c.id )
> OR exists (SELECT  1 FROM email e WHERE e.addr = ? and  e.contact_id=c.id );

hm, how about:
SELECT c.*
FROM contacts c
WHERE  exists  (
  SELECT  1
  FROM phone p
  WHERE p.addr =? AND p.contact_id=c.id
  UNION ALL
  SELECT  1 FROM email e
  WHERE e.addr = ? AND e.contact_id=c.id
);

merlin