PostgreSQL finding queries using an index I want to drop - postgresql

Question

In a PostgreSQL 9.4.0 database I have a busy table with 22 indexes which are larger than the actual data in the table.
Since most of these indexes are for columns which are almost entirely NULL, I've been trying to replace some of them with partial indexes.
One of the columns is: auto_decline_at timestamp without time zone. This has 5453085 NULLS out of a total 5457088 rows.
The partial index replacement is being used, but according to the stats, the old index is also still in use, so I am afraid to drop it.
Selecting from pg_tables I see:
tablename | indexname | num_rows | table_size | index_size | unique | number_of_scans | tuples_read | tuples_fetched
-----------+---------------------------------------+-------------+------------+------------+--------+-----------------+-------------+----------------
jobs | index_jobs_on_auto_decline_at | 5.45496e+06 | 1916 MB | 3123 MB | N | 17056009 | 26506058607 | 26232155810
jobs | index_jobs_on_auto_decline_at_partial | 5.45496e+06 | 1916 MB | 120 kB | N | 6677 | 26850779 | 26679802
And a few minutes later:
tablename | indexname | num_rows | table_size | index_size | unique | number_of_scans | tuples_read | tuples_fetched
-----------+---------------------------------------+-------------+------------+------------+--------+-----------------+-------------+----------------
jobs | index_jobs_on_auto_decline_at | 5.45496e+06 | 1916 MB | 3124 MB | N | 17056099 | 26506058697 | 26232155900
jobs | index_jobs_on_auto_decline_at_partial | 5.45496e+06 | 1916 MB | 120 kB | N | 6767 | 27210639 | 27039623
So number_of_scans is increasing for both of them.
The index definitions:
"index_jobs_on_auto_decline_at" btree (auto_decline_at)
"index_jobs_on_auto_decline_at_partial" btree (auto_decline_at) WHERE auto_decline_at IS NOT NULL
The only relevant query I can see in my logs follows this pattern:
SELECT "jobs".* FROM "jobs" WHERE (jobs.pending_destroy IS NULL OR jobs.pending_destroy = FALSE) AND "jobs"."deleted_at" IS NULL AND (state = 'assigned' AND auto_decline_at IS NOT NULL AND auto_decline_at < '2015-08-17 06:57:22.325324')
EXPLAIN ANALYSE gives me the following plan, which uses the partial index as expected:
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using index_jobs_on_auto_decline_at_partial on jobs (cost=0.28..12.27 rows=1 width=648) (actual time=22.143..22.143 rows=0 loops=1)
Index Cond: ((auto_decline_at IS NOT NULL) AND (auto_decline_at < '2015-08-17 06:57:22.325324'::timestamp without time zone))
Filter: (((pending_destroy IS NULL) OR (NOT pending_destroy)) AND (deleted_at IS NULL) AND ((state)::text = 'assigned'::text))
Rows Removed by Filter: 3982
Planning time: 2.731 ms
Execution time: 22.179 ms
(6 rows)
My questions:
Why is index_jobs_on_auto_decline_at still being used?
Could this same query sometimes use index_jobs_on_auto_decline_at, or is there likely to be another query I am missing?
Is there a way to log the queries which are using index_jobs_on_auto_decline_at?