services/horizon: Add (asset, account_id) index to trust_lines table (#…

…4635)

Adds a new index to `trust_lines` table on `(asset_type, asset_code,
asset_issuer, account_id)` fields.

When filtering by `asset` in `/accounts` endpoint the underlying query was using
`"trust_lines_by_type_code_issuer" btree (asset_type, asset_code, asset_issuer)`
index. This can be slow when querying for a popular asset which (like `USDC`)
because when selected trust lines are found by a DB engine, they later need to
be sorted by account id. A new index allow quick sorting of found trustlines.

### Query plan without a new index

```
 Limit  (cost=2760.42..2787.18 rows=10 width=249) (actual time=725.657..734.633 rows=10 loops=1)
   ->  Nested Loop  (cost=2760.42..7229.44 rows=1670 width=249) (actual time=725.656..734.631 rows=10 loops=1)
         ->  Gather Merge  (cost=2759.86..2950.19 rows=1670 width=57) (actual time=725.622..734.541 rows=10 loops=1)
               Workers Planned: 1
               Workers Launched: 1
               ->  Sort  (cost=1759.85..1762.30 rows=982 width=57) (actual time=719.248..719.275 rows=203 loops=2)
                     Sort Key: trust_lines.account_id
                     Sort Method: external merge  Disk: 8104kB
                     Worker 0:  Sort Method: external merge  Disk: 7472kB
                     ->  Parallel Bitmap Heap Scan on trust_lines  (cost=24.86..1711.05 rows=982 width=57) (actual time=49.144..656.403 rows=118316 loops=2)
                           Recheck Cond: ((asset_type = 1) AND ((asset_code)::text = 'USDC'::text) AND ((asset_issuer)::text = 'GA5ZSEJYB37JRC5AVCIA5MOP4RHTM335X2KGX3IHOJAPP5RE34K4KZVN'::text))
                           Rows Removed by Index Recheck: 1905016
                           Heap Blocks: exact=18001 lossy=87732
                           ->  Bitmap Index Scan on trust_lines_by_type_code_issuer  (cost=0.00..24.44 rows=1670 width=0) (actual time=45.137..45.137 rows=236633 loops=1)
                                 Index Cond: ((asset_type = 1) AND ((asset_code)::text = 'USDC'::text) AND ((asset_issuer)::text = 'GA5ZSEJYB37JRC5AVCIA5MOP4RHTM335X2KGX3IHOJAPP5RE34K4KZVN'::text))
         ->  Index Scan using accounts_pkey on accounts  (cost=0.56..2.56 rows=1 width=192) (actual time=0.008..0.008 rows=1 loops=10)
               Index Cond: ((account_id)::text = (trust_lines.account_id)::text)
 Planning Time: 2.946 ms
 Execution Time: 738.166 ms
(19 rows)

```

### Query plan **with** a new index

```
 Limit  (cost=1.25..27.29 rows=10 width=249) (actual time=0.319..0.376 rows=10 loops=1)
   ->  Nested Loop  (cost=1.25..4351.51 rows=1670 width=249) (actual time=0.318..0.374 rows=10 loops=1)
         ->  Index Only Scan using trust_lines_by_account_type_code_issuer on trust_lines  (cost=0.69..72.26 rows=1670 width=57) (actual time=0.303..0.308 rows=10 loops=1)
               Index Cond: ((asset_type = 1) AND (asset_code = 'USDC'::text) AND (asset_issuer = 'GA5ZSEJYB37JRC5AVCIA5MOP4RHTM335X2KGX3IHOJAPP5RE34K4KZVN'::text))
               Heap Fetches: 0
         ->  Index Scan using accounts_pkey on accounts  (cost=0.56..2.56 rows=1 width=192) (actual time=0.006..0.006 rows=1 loops=10)
               Index Cond: ((account_id)::text = (trust_lines.account_id)::text)
 Planning Time: 2.113 ms
 Execution Time: 0.420 ms
(9 rows)
```

services/horizon/internal/db2/schema/migrations/61_trust_lines_by_account_type_code_issuer.sql

@@ -0,0 +1,9 @@
+-- +migrate Up
+
+CREATE INDEX "trust_lines_by_type_code_issuer_account" ON trust_lines USING btree (asset_type, asset_code, asset_issuer, account_id);
+DROP INDEX "trust_lines_by_type_code_issuer";
+
+-- +migrate Down
+
+DROP INDEX "trust_lines_by_type_code_issuer_account";
+CREATE INDEX "trust_lines_by_type_code_issuer" ON trust_lines USING btree (asset_type, asset_code, asset_issuer);