Postgresql - Clean way to insert records if they don't exist, update if they do - database

Question

Here's my situation. I have a table with a bunch of URLs and crawl
dates associated with them. When my program processes a URL, I want
to INSERT a new row with a crawl date. If the URL already exists, I
want to update the crawl date to the current datetime. With MS SQL or
Oracle I'd probably use a MERGE command for this. With mySQL I'd
probably use the ON DUPLICATE KEY UPDATE syntax.
I could do multiple queries in my program, which may or may not be
thread safe. I could write a SQL function which has various IF...ELSE
logic. However, for the sake of trying out Postgres features I've
never used before, I'm thinking about creating an INSERT rule -
something like this:
CREATE RULE Pages_Upsert AS ON INSERT TO Pages
WHERE EXISTS (SELECT 1 from Pages P where NEW.Url = P.Url)
DO INSTEAD
UPDATE Pages SET LastCrawled = NOW(), Html = NEW.Html WHERE Url = NEW.Url;
This seems to actually work great. It probably loses some points on
the "code readability" standpoint, as someone looking at my code for
the first time would have to magically know about this rule, but I
guess that could be solved with good code commenting and
documentation.
Are there any other drawbacks to this idea, or maybe a "your idea
sucks, you should do it /this/ way instead" comment? I'm on PG 9.0 if
that matters.
UPDATE: Query plan since someone wanted it :)
"Insert (cost=2.79..2.81 rows=1 width=0)"
" InitPlan 1 (returns $0)"
" -> Seq Scan on pages p (cost=0.00..2.79 rows=1 width=0)"
" Filter: ('http://www.foo.com'::text = lower((url)::text))"
" -> Result (cost=0.00..0.01 rows=1 width=0)"
" One-Time Filter: ($0 IS NOT TRUE)"
""
"Update (cost=2.79..5.46 rows=1 width=111)"
" InitPlan 1 (returns $0)"
" -> Seq Scan on pages p (cost=0.00..2.79 rows=1 width=0)"
" Filter: ('http://www.foo.com'::text = lower((url)::text))"
" -> Result (cost=0.00..2.67 rows=1 width=111)"
" One-Time Filter: $0"
" -> Seq Scan on pages (cost=0.00..2.66 rows=1 width=111)"
" Filter: ((url)::text = 'http://www.foo.com'::text)"

Answer 1

Ok, I managed to create a testcase. The result is that the update part is always executed, even on a fresh insert. COPY seems to bypass the rule system.
[For clarity I have put this into a separate reply]
DROP TABLE pages CASCADE;
CREATE TABLE pages
( url VARCHAR NOT NULL PRIMARY KEY
, html VARCHAR
, last TIMESTAMP
);
INSERT INTO pages(url,html,last) VALUES ('www.example.com://page1' , 'meuk1' , '2001-09-18 23:30:00'::timestamp );
CREATE RULE Pages_Upsert AS ON INSERT TO pages
WHERE EXISTS (SELECT 1 from pages P where NEW.url = P.url)
DO INSTEAD (
UPDATE pages SET html=new.html , last = NOW() WHERE url = NEW.url
);
INSERT INTO pages(url,html,last) VALUES ('www.example.com://page2' , 'meuk2' , '2002-09-18 23:30:00':: timestamp );
INSERT INTO pages(url,html,last) VALUES ('www.example.com://page3' , 'meuk3' , '2003-09-18 23:30:00':: timestamp );
INSERT INTO pages(url,html,last) SELECT pp.url || '/added'::text, pp.html || '.html'::text , pp.last + interval '20 years' FROM pages pp;
COPY pages(url,html,last) FROM STDIN;
www.example.com://pageX stdin 2000-09-18 23:30:00
\.
SELECT * FROM pages;
The result:
url | html | last
-------------------------------+------------+----------------------------
www.example.com://page1 | meuk1 | 2001-09-18 23:30:00
www.example.com://page2 | meuk2 | 2011-09-18 23:48:30.775373
www.example.com://page3 | meuk3 | 2011-09-18 23:48:30.783758
www.example.com://page1/added | meuk1.html | 2011-09-18 23:48:30.792097
www.example.com://page2/added | meuk2.html | 2011-09-18 23:48:30.792097
www.example.com://page3/added | meuk3.html | 2011-09-18 23:48:30.792097
www.example.com://pageX | stdin | 2000-09-18 23:30:00
(7 rows)
UPDATE: Just to prove it can be done:
INSERT INTO pages(url,html,last) VALUES ('www.example.com://page1' , 'meuk1' , '2001-09-18 23:30:00'::timestamp );
CREATE VIEW vpages AS (SELECT * from pages);
CREATE RULE Pages_Upsert AS ON INSERT TO vpages
DO INSTEAD (
UPDATE pages p0
SET html=NEW.html , last = NOW() WHERE p0.url = NEW.url
;
INSERT INTO pages (url,html,last)
SELECT NEW.url, NEW.html, NEW.last
WHERE NOT EXISTS ( SELECT * FROM pages p1 WHERE p1.url = NEW.url)
);
CREATE RULE Pages_Indate AS ON UPDATE TO vpages
DO INSTEAD (
INSERT INTO pages (url,html,last)
SELECT NEW.url, NEW.html, NEW.last
WHERE NOT EXISTS ( SELECT * FROM pages p1 WHERE p1.url = OLD.url)
;
UPDATE pages p0
SET html=NEW.html , last = NEW.last WHERE p0.url = NEW.url
;
);
INSERT INTO vpages(url,html,last) VALUES ('www.example.com://page2' , 'meuk2' , '2002-09-18 23:30:00':: timestamp );
INSERT INTO vpages(url,html,last) VALUES ('www.example.com://page3' , 'meuk3' , '2003-09-18 23:30:00':: timestamp );
INSERT INTO vpages(url,html,last) SELECT pp.url || '/added'::text, pp.html || '.html'::text , pp.last + interval '20 years' FROM vpages pp;
UPDATE vpages SET last = last + interval '-10 years' WHERE url = 'www.example.com://page1' ;
-- Copy does NOT work on views
-- COPY vpages(url,html,last) FROM STDIN;
-- www.example.com://pageX stdin 2000-09-18 23:30:00
-- \.
SELECT * FROM vpages;
Result:
INSERT 0 1
INSERT 0 1
INSERT 0 3
UPDATE 1
url | html | last
-------------------------------+------------+---------------------
www.example.com://page2 | meuk2 | 2002-09-18 23:30:00
www.example.com://page3 | meuk3 | 2003-09-18 23:30:00
www.example.com://page1/added | meuk1.html | 2021-09-18 23:30:00
www.example.com://page2/added | meuk2.html | 2022-09-18 23:30:00
www.example.com://page3/added | meuk3.html | 2023-09-18 23:30:00
www.example.com://page1 | meuk1 | 1991-09-18 23:30:00
(6 rows)
The view is necessary to prevent the rewrite system to go into recursion.
Construction of a DELETE rule is left as an exercise to the reader.

Answer 2

Some good points from someone who should know it or be very near to someone like that ;-)
What are PostgreSQL RULEs good for?
Short story:
Do the rules work well with SERIAL and BIGSERIAL ?
Do the rules work well with the RETURNING clauses of INSERT and UPDATE ?
Do the rules work well with stuff like random()?
All these things boils down to the fact, that the rule system is not row driven but transforms your statements in a way you never imagine.
Do yourself and your team mates a favour and stop using roles for things like that.
Edit: Your problem is well discussed in the PostgreSQL community. Search keywords are: MERGE, UPSERT.

Answer 3

I don't know if this gets too subjective but what I think about your solution is: It's all about semantics. When I do an insert, I expect an insert and not some fancy logic that maybe does an insert but maybe not. Indeed that's what functions are for.
At first I'd try checking for the URL in your program and then choosing whether to insert or update. If that turned out to be too slow, I'd use a function. If you name it like insert_or_update_url, you automatically get some documentation for free. The rewrite rule requires you to have some implicit knowledge and I generally try to avoid that.
On the plus side: If someone copies the data but forgets rules and functions, your solution might break silently (but that may depend on other constraints), but a missing function goes down screaming. Don't get me wrong, I think your solution is very creative and smart. Just a bit too obscure for my taste.

Answer 4

There's an example of implementing upsert / merge using simple function in Postgres documentation.
Never use rules — they're evil.

Answer 5

You cannot refer to other tables than old an new in the rule qualification.
You should instead do this in the rule body.
This is all because the rule is just a way to inform the rewrite system about what transformations it should and should not perform. Rules are not triggers, executing for every row, but they give the query planner a fine massage and ask it nicely to rewrite the plan.
From the docs:
What is a rule qualification? It is a restriction that tells when the actions of the rule should be done and when not. This qualification can only reference the pseudorelations NEW and/or OLD, which basically represent the relation that was given as object (but with a special meaning).