Fastest check if row exists in PostgreSQL

Question

I have a bunch of rows that I need to insert into table  but these inserts are always done in batches  So I want to check if a single row from the batch exists in the table because then I know they all were inserted   So its not a primary key check  but shouldn t matter too much  I would like to only check single row so count    probably isn t good  so its something like exists I guess   But since I m fairly new to PostgreSQL I d rather ask people who know   My batch contains rows with following structure   userid   rightid   remaining count   So if table contains any rows with provided userid it means they all are present there

User · Answer

If you think about the performace  may be you can  use  PERFORM  in a function just like this    PERFORM 1 FROM skytf test 2 WHERE id i LIMIT 1    IF FOUND THEN       RAISE NOTICE   found record id     i      ELSE       RAISE NOTICE   not found record id     i     END IF

User · Answer

Use the EXISTS key word for TRUE   FALSE return   select exists select 1 from contact where id 12

User · Answer

SELECT 1 FROM user right where userid     LIMIT 1   If your resultset contains a row then you do not have to insert  Otherwise insert your records

User · Answer

INSERT INTO target  userid  rightid  count     SELECT userid  rightid  count    FROM batch   WHERE NOT EXISTS       SELECT   FROM target t2  batch b2     WHERE t2 userid   b2 userid     --     other keyfields                          BTW  if you want the whole batch to fail in case of a duplicate  then  given a primary key constraint   INSERT INTO target  userid  rightid  count   SELECT userid  rightid  count  FROM batch         will do exactly what you want  either it succeeds  or it fails

User · Answer

How about simply   select 1 from tbl where userid   123 limit 1    where 123 is the userid of the batch that you re about to insert   The above query will return either an empty set or a single row  depending on whether there are records with the given userid   If this turns out to be too slow  you could look into creating an index on tbl userid      if even a single row from batch exists in table  in that case I   don t have to insert my rows because I know for sure they all were   inserted    For this to remain true even if your program gets interrupted mid-batch  I d recommend that you make sure you manage database transactions appropriately  i e  that the entire batch gets inserted within a single transaction

User · Answer

I would like to propose another thought to specifically address your sentence   So I want to check if a single row from the batch exists in the table because then I know they all were inserted    You are making things efficient by inserting in  batches  but then doing existence checks one record at a time  This seems counter intuitive to me  So when you say  inserts are always done in batches  I take it you mean you are inserting multiple records with one insert statement  You need to realize that Postgres is ACID compliant  If you are inserting multiple records  a batch of data  with one insert statement  there is no need to check if some were inserted or not  The statement either passes or it will fail  All records will be inserted or none   On the other hand  if your C  code is simply doing a  set  separate insert statements  for example  in a loop  and in your mind  this is a  batch     then you should not in fact describe it as  inserts are always done in batches   The fact that you expect that part of what you call a  batch   may actually not be inserted  and hence feel the need for a check  strongly suggests this is the case  in which case you have a more fundamental problem  You need change your paradigm to actually insert multiple records with one insert  and forego checking if the individual records made it   Consider this example   CREATE TABLE temp test       id SERIAL PRIMARY KEY      sometext TEXT      userid INT      somethingtomakeitfail INT unique   -- insert a batch of 3 rows    INSERT INTO temp test  sometext  userid  somethingtomakeitfail  VALUES   foo   1  1     bar   2  2     baz   3  3     -- inspect the data of what we inserted SELECT   FROM temp test    -- this entire statement will fail    no need to check which one made it INSERT INTO temp test  sometext  userid  somethingtomakeitfail  VALUES   foo   2  4     bar   2  5     baz   3  3   --  lt  lt -- deliberately simulate a failure     -- check it     everything is the same from the last successful insert    -- no need to check which records from the 2nd insert may have made it in SELECT   FROM temp test   This is in fact the paradigm for any ACID compliant DB    not just Postgresql   In other words you are better off if you fix your  batch  concept and avoid having to do any row by row checks in the first place

User · Answer

select true from tablename where condition limit 1    I believe that this is the query that postgres uses for checking foreign keys   In your case  you could do this in one go too   insert into yourtable select  userid   rightid   count where not  select true from yourtable where userid    userid limit 1

User · Answer

as  MikeM pointed out   select exists select 1 from contact where id 12    with index on contact  it can usually reduce time cost to 1 ms   CREATE INDEX index contact on contact id

[sql] Fastest check if row exists in PostgreSQL

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