检查Postgres表中是否存在记录

Question

I have to read a CSV every 20 seconds. Each CSV contains min. of 500 to max. 60000 lines. I have to insert the data in a Postgres table, but before that I need to check if the items have already been inserted, because there is a high probability of getting duplicate item. The field to check for uniqueness is also indexed.

So, I read the file in chunks and use the IN clause to get the items already in the database.

Is there a better way of doing it?

Answer 1

This should perform well:

CREATE TEMP TABLE tmp AS SELECT * FROM tbl LIMIT 0 -- copy layout, but no data

COPY tmp FROM '/absolute/path/to/file' FORMAT csv;

INSERT INTO tbl
SELECT tmp.*
FROM   tmp
LEFT   JOIN tbl USING (tbl_id)
WHERE  tbl.tbl_id IS NULL;

DROP TABLE tmp; -- else dropped at end of session automatically

Closely related to this answer .

Answer 2

First just for completeness I changed Erwin's code to use except

CREATE TEMP TABLE tmp AS SELECT * FROM tbl LIMIT 0 -- copy layout, but no data
COPY tmp FROM '/absolute/path/to/file' FORMAT csv;

INSERT INTO tbl
SELECT tmp.*
FROM   tmp
except
select *
from tbl

DROP TABLE tmp;

Then I resolved to test it myself. I tested it in 9.1 with a mostly untouched postgresql.conf . The target table contains 10 million rows and the origin table 30 thousand. 15 thousand already exists in the target table.

create table tbl (id integer primary key)
;
insert into tbl
select generate_series(1, 10000000)
;
create temp table tmp as select * from tbl limit 0
;
insert into tmp
select generate_series(9985000, 10015000)
;

I asked for the explain of the select part only. The except version:

explain
select *
from tmp
except
select *
from tbl
;
                                       QUERY PLAN                                       
----------------------------------------------------------------------------------------
 HashSetOp Except  (cost=0.00..270098.68 rows=200 width=4)
   ->  Append  (cost=0.00..245018.94 rows=10031897 width=4)
         ->  Subquery Scan on "*SELECT* 1"  (cost=0.00..771.40 rows=31920 width=4)
               ->  Seq Scan on tmp  (cost=0.00..452.20 rows=31920 width=4)
         ->  Subquery Scan on "*SELECT* 2"  (cost=0.00..244247.54 rows=9999977 width=4)
               ->  Seq Scan on tbl  (cost=0.00..144247.77 rows=9999977 width=4)
(6 rows)

The outer join version:

explain
select *
from 
    tmp
    left join
    tbl using (id)
where tbl.id is null
;
                                QUERY PLAN                                
--------------------------------------------------------------------------
 Nested Loop Anti Join  (cost=0.00..208142.58 rows=15960 width=4)
   ->  Seq Scan on tmp  (cost=0.00..452.20 rows=31920 width=4)
   ->  Index Scan using tbl_pkey on tbl  (cost=0.00..7.80 rows=1 width=4)
         Index Cond: (tmp.id = id)
(4 rows)