When you create a new company in a database that has a lot of existing
account move lines and you attempt to open a PoS register from the list
view, `_compute_company_has_template` checks `_existing_accounting` for
the new company and wil run a sequential scan on the entire account_move_line
table followed by a nested loop as the query planner assumes AMLs company_ids
will be roughly evenly distributed.

This is not the case in a new company that has no/very few AMLs.

This is because the query ran is:
`SELECT COUNT(*) FROM
    (SELECT FROM "account_move_line"
    WHERE (
    "account_move_line"."company_id" IN
        (SELECT "res_company"."id" FROM
        "res_company" WHERE
        ("res_company"."parent_path" LIKE '3/%')
    ))
LIMIT 1)`

and the values of company_id being searched for aren't known until the
subquery runs.

Running a query more like
`SELECT COUNT(*) FROM
account_move_line
WHERE company_id IN (%s)`
is much faster

Since res_company will always be a smaller table, we can do the inexpensive
search first and then pass in the values so Postgres can do a cheaper
search and return faster.

Benchmark time of `_existing_accounting`:
| Company 1 AML count | Company 2 AML count | Pre-fix | Post-fix | Multiplier |
|---|---|---|---|---|
| 10,000,000 | 0 | 700 Milliseconds | 500 Microseconds | 1,400x |
| 20,000,000 | 0 | 1.35 Seconds | 1 Millisecond | 1,350x |
| 20,000,000 | 20,000,000 | 2 Milliseconds |1.3 Milliseconds | 1.5x |
| 50,000,000 | 0 | 3.25 Seconds | 1 Millisecond | 3,250x |
| 100,000,000 | 0 | 5.3 Seconds | 1.3 Milliseconds | 4,075x |

Query Plan Before:
```
"Aggregate  (cost=0.08..0.09 rows=1 width=8) (actual time=5573.001..5573.002 rows=1.00 loops=1)"
"  Buffers: shared read=571435"
"  ->  Limit  (cost=0.00..0.08 rows=1 width=0) (actual time=5572.995..5572.997 rows=0.00 loops=1)"
"        Buffers: shared read=571435"
"        ->  Nested Loop  (cost=0.00..https://github.com/odoo/odoo/commit/1571439c0c70c1f1dc3229421e696b97ce1678f8.33 rows=20000086 width=0) (actual time=5572.988..5572.989 rows=0.00 loops=1)"
"              Join Filter: (account_move_line.company_id = res_company.id)"
"              Buffers: shared read=571435"
"              ->  Seq Scan on account_move_line  (cost=0.00..971432.72 rows=40000172 width=4) (actual time=0.432..1913.934 rows=40000000.00 loops=1)"
"                    Buffers: shared read=571431"
"              ->  Materialize  (cost=0.00..4.03 rows=1 width=4) (actual time=0.000..0.000 rows=0.00 loops=40000000)"
"                    Storage: Memory  Maximum Storage: 17kB"
"                    Buffers: shared read=4"
"                    ->  Seq Scan on res_company  (cost=0.00..4.03 rows=1 width=4) (actual time=0.785..0.785 rows=0.00 loops=1)"
"                          Filter: ((parent_path)::text ~~ '3/%'::text)"
"                          Rows Removed by Filter: 2"
"                          Buffers: shared read=4"
"Planning:"
"  Buffers: shared hit=574 read=77"
"Planning Time: 15.323 ms"
"Execution Time: 5573.060 ms"
```
Query Plan After:
```
"Aggregate  (cost=4.46..4.47 rows=1 width=8) (actual time=1.972..1.973 rows=1.00 loops=1)"
"  Buffers: shared read=3"
"  ->  Limit  (cost=0.44..4.46 rows=1 width=0) (actual time=1.967..1.968 rows=0.00 loops=1)"
"        Buffers: shared read=3"
"        ->  Index Only Scan using account_move_line__company_id_index on account_move_line  (cost=0.44..4.46 rows=1 width=0) (actual time=1.965..1.966 rows=0.00 loops=1)"
"              Index Cond: (company_id = 3)"
"              Heap Fetches: 0"
"              Index Searches: 1"
"              Buffers: shared read=3"
"Planning:"
"  Buffers: shared hit=3"
"Planning Time: 0.219 ms"
"Execution Time: 1.998 ms"
```
opw-6513885

Forward-Port-Of: odoo/odoo#285096