Postgresql deadlock issue - postgresql

Question

I have received the following deadlock error in pg_log:
2016-01-15 09:52:48.648 EST,"name","name",11694,"ip:40273",56988e35.2dae,1,"UPDATE",2016-01-15 01:14:13 EST,10/3886,49775,ERROR,40P01,
"deadlock detected",
"Process 11694 waits for ShareLock on transaction 49774; blocked by process 11685.
Process 11685 waits for ShareLock on transaction 49775; blocked by process 11694.
Process 11694: update bb_batter_season_stat set a_field=a_field+$1, ab_int=ab_int+$2, abvsl=abvsl+$3, abvsr=abvsr+$4, bbvsl=bbvsl+$5, bbvsr=bbvsr+$6, cs=cs+$7,
cs_field=cs_field+$8, ... where bb_players_id=$53
Process 11685: update bb_batter_season_stat set a_field=a_field+$1, ab_int=ab_int+$2, abvsl=abvsl+$3, abvsr=abvsr+$4, bbvsl=bbvsl+$5, bbvsr=bbvsr+$6, cs=cs+$7,
cs_field=cs_field+$8, ... where bb_players_id=$53","See server log for query details.",,,,
"update bb_batter_season_stat set a_field=a_field+$1, ab_int=ab_int+$2, abvsl=abvsl+$3,
abvsr=abvsr+$4, bbvsl=bbvsl+$5, bbvsr=bbvsr+$6, cs=cs+$7, cs_field=cs_field+$8, ... where bb_players_id=$53",,,""
I can't understand why is it happening. Two processes running same query and deadlock happens.
The table schema is:
CREATE TABLE bb_batter_season_stat (
id SERIAL NOT NULL ,
bb_players_id INTEGER NOT NULL ,
G SMALLINT ,
ABvsL SMALLINT ,
ABvsR SMALLINT ,
RvsL SMALLINT ,
RvsR SMALLINT ,
HvsL SMALLINT ,
HvsR SMALLINT ,
d2BvsL SMALLINT ,
d2BvsR SMALLINT ,
...
PRIMARY KEY(id) ,
FOREIGN KEY(bb_players_id) REFERENCES bb_players(id) );
CREATE INDEX bb_batter_season_stat_FKIndex1 ON bb_batter_season_stat (bb_players_id);

Answer 1

I suppose that you have at least 2 records in bb_batter_season_stat that have the same bb_players_id.
Transaction one locked for update the first record.
Transaction two locked the second.
Transaction one tried to lock the the second, but was blocked waiting for transaction two.
Transaction two tried to lock the first record, but was blocked waiting for transaction one, creating a deadlock.
To avoid this you should force locking of the records in the order of primary key. For example using select for update:
with ids as (
select id
from bb_batter_season_stat
where bb_players_id=?
order by id
for update
)
update bb_batter_season_stat
set a_field=a_field+$1, ab_int=ab_int+$2, …
where id in (select id from ids);