Postgres select on large (15m rows) table extremely slow, even with index

Question

I'm trying to run EXPLAIN ANALYZE but it simply won't finish because it's so slow. If it does, I'll post the results, but for now, here is the EXPLAIN.

Query:

EXPLAIN SELECT
    *
FROM
    "Posts" AS "Post"
WHERE
    (
        "Post"."featurePostOnDate" > '2020-06-25 19:28:07.816 +00:00'
        OR (
            "Post"."featurePostOnDate" IS NULL
            AND "Post"."userId" IN (6863684)
        )
    )
AND "Post"."private" IS NULL
ORDER BY
    "Post"."featurePostOnDate" DESC NULLS LAST,
    "Post"."createdAt" DESC NULLS LAST
LIMIT 10;

Result:

Limit  (cost=0.56..110.92 rows=10 width=1136)
  ->  Index Scan using posts_updated_following_feed_idx on "Posts" "Post"  (cost=0.56..284949.60 rows=25819 width=1136)
        Filter: (("featurePostOnDate" > '2020-06-25 19:28:07.816+00'::timestamp with time zone) OR (("featurePostOnDate" IS NULL) AND ("userId" = 6863684)))

Index:

CREATE INDEX  "posts_updated_following_feed_idx" ON "public"."Posts" USING btree (
    "featurePostOnDate" DESC NULLS LAST,
    "createdAt" DESC NULLS LAST
)
WHERE
    private IS NULL;

Answer 1

You would need to write it as two separate queries, one for each branch of the OR. Apply the limit to each query, then combine them and apply the limit again jointly. But if the first branch finds ten rows, the second one doesn't need to run at all as all non-NULL dates already come first.