Rules, Windows and ORDER BY

Hello List,

I have a simple table of keys and values which periodically
receives updated values. It's desirable to keep older values
but, most of the time, we query only for the latest value of a
particular key.

CREATE TABLE kv
( k bytea NOT NULL,
at timestamptz NOT NULL,
realm bytea NOT NULL,
v bytea NOT NULL );
CREATE INDEX ON kv USING hash(k);
CREATE INDEX ON kv (t);
CREATE INDEX ON kv USING hash(realm);

SELECT * FROM kv WHERE k = $1 AND realm = $2 ORDER BY at DESC LIMIT 1;

It would be nice to encapsulate this common query with a VIEW;
for example:

CREATE VIEW kv_new AS
SELECT * FROM kv WHERE at =
( SELECT at FROM kv AS _
WHERE _.k = kv.k AND _.realm = kv.realm
ORDER BY at DESC LIMIT 1 );

I tried partition functions, at first, but they were really very
slow. This view is pretty sprightly but has a more complicated
plan than the original query, which only has a sort followed by
an index scan, and is consequently not as fast. Please find the
plans below my signature.

Ideally, I'd be able to create a rule where the ORDER BY and
LIMIT were simply appended to whatever SELECT was given; but I
am at a loss as to how to do that. Creating a VIEW with the
order and limit just gives me a table with one row in it (of
course).

Is there something better than a sub-select here? I tried using
one with max(at) but it's not noticeably faster. I would be
interested to see how others have approached this kind of log-
-structured storage in Postgres. The window functions make,
alas, no use of indexes.

--
Jason Dusek
pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B

EXPLAIN (COSTS FALSE, FORMAT YAML)
SELECT * FROM kv WHERE k = ... AND realm = ... ORDER BY at LIMIT 1;
-[ RECORD 1 ]-----------------------------------------
QUERY PLAN | - Plan:
| Node Type: "Limit"
| Plans:
| - Node Type: "Sort"
| Parent Relationship: "Outer"
| Sort Key:
| - "at"
| Plans:
| - Node Type: "Index Scan"
| Parent Relationship: "Outer"
| Scan Direction: "NoMovement"
| Index Name: "kv_k_idx"
| Relation Name: "kv"
| Alias: "kv"
| Index Cond: "(k = ...)"
| Filter: "(realm = ...)"

EXPLAIN (COSTS FALSE, FORMAT YAML)
SELECT * FROM kv_new WHERE k = ... AND realm = ...;
-[ RECORD 1 ]-----------------------------------------------------
QUERY PLAN | - Plan:
| Node Type: "Index Scan"
| Scan Direction: "NoMovement"
| Index Name: "kv_k_idx"
| Relation Name: "kv"
| Alias: "kv"
| Index Cond: "(k = ...)"
| Filter: "((realm = ...) AND (at = (SubPlan 1)))"
| Plans:
| - Node Type: "Limit"
| Parent Relationship: "SubPlan"
| Subplan Name: "SubPlan 1"
| Plans:
| - Node Type: "Sort"
| Parent Relationship: "Outer"
| Sort Key:
| - "_.at"
| Plans:
| - Node Type: "Index Scan"
| Parent Relationship: "Outer"
| Scan Direction: "NoMovement"
| Index Name: "kv_k_idx"
| Relation Name: "kv"
| Alias: "_"
| Index Cond: "(k = kv.k)"
| Filter: "(realm = kv.realm)"