> My recollection in postgres anyway is that low cardinality indexes aren't useful, because it doesn't take into account which side of the 1/99% you're on when determining to use the index.

It does take that into account. Demo:

    =# CREATE TABLE low_cardinality AS SELECT generate_series(-10, 1000000) < 0 AS col;
    =# CREATE INDEX ON low_cardinality (col);
    =# ANALYZE low_cardinality;
    =# EXPLAIN SELECT * FROM low_cardinality WHERE col = false;
    ┌─────────────────────────────────────────────────────────────────────────┐
    │                               QUERY PLAN                                │
    ├─────────────────────────────────────────────────────────────────────────┤
    │ Seq Scan on low_cardinality  (cost=0.00..14425.11 rows=1000011 width=1) │
    │   Filter: (NOT col)                                                     │
    └─────────────────────────────────────────────────────────────────────────┘
    (2 rows)

    =# EXPLAIN SELECT * FROM low_cardinality WHERE col = true;
    ┌────────────────────────────────────────────────────────────────────────────────────────────────────┐
    │                                             QUERY PLAN                                             │
    ├────────────────────────────────────────────────────────────────────────────────────────────────────┤
    │ Index Only Scan using low_cardinality_col_idx on low_cardinality  (cost=0.42..4.44 rows=1 width=1) │
    │   Index Cond: (col = true)                                                                         │
    └────────────────────────────────────────────────────────────────────────────────────────────────────┘
    (2 rows)