fix(pgvector): make doc deletion query faster and use chunking

[kyteinsky]

CI logging for slow queries has also been enabled, not sure if we will see that in the CI though.

Sample output for the slow deletion query where a missing index on the source_id foreign key in access_list table was the culprit.
Calculated time: 3.495 + 0.310 + 0.129 = 3.934 ms
Actual time: 201177.123 ms or 201 s

        Query Text: DELETE FROM docs WHERE docs.source_id IN ($1::VARCHAR, $2::VARCHAR, ..., $275::VARCHAR) RETURNING docs.chunks
        Query Parameters: ...
        Delete on docs  (cost=1126.32..2018.25 rows=275 width=6) (actual time=0.192..3.495 rows=218 loops=1)
    ->  Bitmap Heap Scan on docs  (cost=1126.32..2018.25 rows=275 width=6) (actual time=0.144..0.310 rows=218 loops=1)
                Recheck Cond: ((source_id)::text = ANY ('{"files__default: 20392","files__default: 23092", ... }'::text[]))
                Heap Blocks: exact=25
                ->  Bitmap Index Scan on docs_pkey  (cost=0.00..1125.56 rows=275 width=0) (actual time=0.129..0.129 rows=218 loops=1)
                      Index Cond: ((source_id)::text = ANY ('{"files__default: 20392", ...
2026-03-19 11:28:59.760 UTC [6703] LOG:  duration: 201177.123 ms  execute <unnamed>: DELETE FROM docs WHERE docs.source_id IN ($1::VARCHAR, $2::VARCHAR, ..., $275::VARCHAR) RETURNING docs.chunks
2026-03-19 11:28:59.760 UTC [6703] DETAIL:  Parameters: $1 = 'files__default: 20392', $2 = ...

(put the chunking part in a different PR)

[kyteinsky]

DB migration needs to be done for this to happen on existing installations.

[kyteinsky]

verified again the index actually does improve things:
before:

ccb=# EXPLAIN ANALYZE DELETE FROM docs WHERE source_id = 'files__default: 100';
                                                            QUERY PLAN                                                             
-----------------------------------------------------------------------------------------------------------------------------------
 Delete on docs  (cost=0.42..8.44 rows=0 width=0) (actual time=1.434..1.435 rows=0 loops=1)
   ->  Index Scan using source_id_modified_idx on docs  (cost=0.42..8.44 rows=1 width=6) (actual time=0.841..0.844 rows=1 loops=1)
         Index Cond: ((source_id)::text = 'files__default: 100'::text)
 Planning Time: 0.069 ms
 Trigger for constraint access_list_source_id_fkey: time=923.137 calls=1
 Execution Time: 924.593 ms
(6 rows)

after:

ccb=# EXPLAIN ANALYZE DELETE FROM docs WHERE source_id = 'files__default: 101';
                                                            QUERY PLAN                                                             
-----------------------------------------------------------------------------------------------------------------------------------
 Delete on docs  (cost=0.42..8.44 rows=0 width=0) (actual time=0.098..0.099 rows=0 loops=1)
   ->  Index Scan using source_id_modified_idx on docs  (cost=0.42..8.44 rows=1 width=6) (actual time=0.062..0.065 rows=1 loops=1)
         Index Cond: ((source_id)::text = 'files__default: 101'::text)
 Planning Time: 0.205 ms
 Trigger for constraint access_list_source_id_fkey: time=0.414 calls=1
 Execution Time: 0.563 ms
(6 rows)

the difference in time to pay attention to is Trigger for constraint access_list_source_id_fkey: time
without index: 923.137
with index: 0.414

index can be manually created like so: CREATE INDEX idx_access_list_source_id ON access_list (source_id);