Incremental DISTINCT / GROUP BY operation - sql

Question

I have a simple two-stage SQL query that operators on two tables A and B, where I use a sub-select to retrieve a number of IDs of table A that are stored as foreign keys in B, using a (possibly complex) query on table B (and possibly other joined tables). Then, I want to simply return the first x IDs of A. I tried using a query like this:
SELECT sq.id
FROM (
SELECT a_id AS id, created_at
FROM B
WHERE ...
ORDER BY created_at DESC
) sq
GROUP BY sq.id
ORDER BY max(sq.created_at) DESC
LIMIT 10;
which is quite slow as Postgres seems to perform the GROUP BY / DISTINCT operation on the whole result set before limiting it. If I LIMIT the sub-query (e.g. to 100), the performance is just fine (as I'd expect), but of course it's no longer guaranteed that there will be at least 10 distinct a_id values in the resulting rows of sq.
Similarly, the query
SELECT a_id AS id
FROM B
WHERE ...
GROUP BY id
ORDER BY max(created_at) DESC
LIMIT 10
is quite slow as Postgres seems to perform a sequential scan on B instead of using an (existing) index. If I remove the GROUP BY clause it uses the index just fine.
The data in table B is such that most rows contain different a_ids, hence even without the GROUP BY most of the returned IDs will be different. The goal I pursue with the grouping is to assure that the result set always contains a given number of entries from A.
Is there a way to perform an "incremental DISTINCT / GROUP BY"? In my naive thinking it would suffice for Postgres to produce result rows and group them incrementally until it reaches the number specified by LIMIT, which in most cases should be nearly instantaneous as most a_id values are different. I tried various ways to query the data but so far I didn't find anything that works reliably.
The Postgres version is 9.6, the data schema as follows:
Table "public.a"
Column | Type | Modifiers
--------+-------------------+------------------------------------------------
id | bigint | not null default nextval('a_id_seq'::regclass)
bar | character varying |
Indexes:
"a_pkey" PRIMARY KEY, btree (id)
"ix_a_bar" btree (bar)
Referenced by:
TABLE "b" CONSTRAINT "b_a_id_fkey" FOREIGN KEY (a_id) REFERENCES a(id)
Table "public.b"
Column | Type | Modifiers
------------+-----------------------------+--------------------------------------------------
id | bigint | not null default nextval('b_id_seq'::regclass)
foo | character varying |
a_id | bigint | not null
created_at | timestamp without time zone |
Indexes:
"b_pkey" PRIMARY KEY, btree (id)
"ix_b_created_at" btree (created_at)
"ix_b_foo" btree (foo)
Foreign-key constraints:
"b_a_id_fkey" FOREIGN KEY (a_id) REFERENCES a(id)

Answer 1

This problem is much more complex than it might seem at a first glance.
If ...
your criteria are not very selective (much more than 10 distinct a_id qualify)
you don't have many duplicate a_id in table B (like you stated)
then there is a very fast way.
To simplify a bit I assume created_at is also defined NOT NULL, or you need to do more.
WITH RECURSIVE top10 AS (
( -- extra parentheses required
SELECT a_id, ARRAY[a_id] AS id_arr, created_at
FROM b
WHERE ... -- your other filter conditions here
ORDER BY created_at DESC, a_id DESC -- both NOT NULL
LIMIT 1
)
UNION ALL -- UNION ALL, not UNION, since we exclude dupes a priori
(
SELECT b.a_id, id_arr || b.a_id, b.created_at
FROM top10 t
JOIN b ON (b.created_at, b.a_id)
< (t.created_at, t.a_id) -- comparing ROW values
AND b.a_id <> ALL (t.id_arr)
WHERE ... -- repeat conditions
ORDER BY created_at DESC, a_id DESC
LIMIT 1
)
)
SELECT a_id
FROM top10
LIMIT 10;
Ideally supported by an index on (created_at DESC, a_id DESC) (or just (created_at, a_id)).
Depending on your other WHERE conditions, other (partial?) indexes may serve even better.
This is particularly efficient for a small result set. Else, and depending on various other details, other solutions may be faster.
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Answer 2

The only way the planner has a chance to avoid sorting the whole table is if you have an index on the complete ORDER BY clause.
Then an index scan can be chosen to get the correct ordering, and the first ten result rows may be found quickly.