Bad plan chosen for union all - Mailing list pgsql-performance

I'm on PostgreSQL 9.6.5 and getting an awkwardly bad plan chosen for my query. I want to do: select investments.id, cim.yield FROM contributions JOIN investments ON contributions.investment_id = investments.id JOIN contribution_investment_metrics_view cim ON cim.investment_id = investments.id WHERE contributions.id IN ('\x58c9c0d3ee944c48b32f814d', '\x11') Where contribution_investment_metrics_view is morally select investment_id, first(val) from (select * from contribution_metrics UNION ALL select * from investment_metrics) group by id Typically, querying this view is very fast since I have indexes in both component queries, leading to a very tight plan: Sort Key: "*SELECT* 1".metric -> Subquery Scan on "*SELECT* 1" (cost=14.68..14.68 rows=1 width=26) (actual time=0.043..0.044 rows=2 loops=1) -> Sort (cost=14.68..14.68 rows=1 width=42) (actual time=0.042..0.043 rows=2 loops=1) Sort Key: cm.metric, cm.last_update_on DESC Sort Method: quicksort Memory: 25kB -> Nested Loop (cost=0.14..14.68 rows=1 width=42) (actual time=0.032..0.034 rows=2 loops=1) -> Index Scan using contributions_investment_id_idx on contributions (cost=0.08..4.77 rows=2 width=26) (actual time=0.026..0.027 rows=1 loops=1) Index Cond: (investment_id = $1) -> Index Only Scan using contribution_metrics_contribution_id_metric_last_update_on_idx on contribution_metrics cm (cost=0.06..4.95 rows=2 width=34) (actual time=0.005..0.006 r Index Cond: (contribution_id = contributions.id) Heap Fetches: 2 -> Subquery Scan on "*SELECT* 2" (cost=0.08..5.86 rows=3 width=26) (actual time=0.008..0.008 rows=3 loops=1) -> Index Only Scan using investment_metrics_investment_id_metric_last_updated_on_idx on investment_metrics im (cost=0.08..5.85 rows=3 width=42) (actual time=0.008..0.008 rows=3 loops=1) Index Cond: (investment_id = $1) Heap Fetches: 3 Unfortunately, when I try to query this view in the larger query above, I get a *much* worse plan for this view, leading to >1000x degradation in performance: -> Append (cost=10329.18..26290.92 rows=482027 width=26) (actual time=90.157..324.544 rows=482027 loops=1) -> Subquery Scan on "*SELECT* 1" (cost=10329.18..10349.44 rows=5788 width=26) (actual time=90.157..91.207 rows=5788 loops=1) -> Sort (cost=10329.18..10332.08 rows=5788 width=42) (actual time=90.156..90.567 rows=5788 loops=1) Sort Key: contributions_1.investment_id, cm.metric, cm.last_update_on DESC Sort Method: quicksort Memory: 645kB -> Hash Join (cost=105.62..10256.84 rows=5788 width=42) (actual time=1.924..85.913 rows=5788 loops=1) Hash Cond: (contributions_1.id = cm.contribution_id) -> Seq Scan on contributions contributions_1 (cost=0.00..9694.49 rows=351495 width=26) (actual time=0.003..38.794 rows=351495 loops=1) -> Hash (cost=85.36..85.36 rows=5788 width=34) (actual time=1.907..1.907 rows=5788 loops=1) Buckets: 8192 Batches: 1 Memory Usage: 453kB -> Seq Scan on contribution_metrics cm (cost=0.00..85.36 rows=5788 width=34) (actual time=0.003..0.936 rows=5788 loops=1) -> Subquery Scan on "*SELECT* 2" (cost=0.08..15941.48 rows=476239 width=26) (actual time=0.017..203.006 rows=476239 loops=1) -> Index Only Scan using investment_metrics_investment_id_metric_last_updated_on_idx1 on investment_metrics im (cost=0.08..14512.76 rows=476239 width=42) (actual time=0.016..160.410 rows=476239 l Heap Fetches: 476239 I've played around with a number of solutions (including lateral joins) and the closest I can come is: select investment_id from contribution_investment_metrics where investment_id = ( select investments.id from investments join contributions on investments.id = contributions.investment_id where contributions.id = '\x58c9c0d3ee944c48b32f814d' ) This doesn't really work for my purposes, since I want to project columns from contributions and investments and I want to run this query on "up to a handful" contributions at once (maybe more than one, never more than 100). I'm on PostgreSQL 9.6.5. Schema and full explain analyzes: https://gist.github.com/awreece/28c359c6d834717ab299665022b19fd6 I don't think it's relevant, but since https://wiki.postgresql.org/wiki/SlowQueryQuestions asks -- I'm running in Heroku. What are my options here? Currently, I'm planning to avoid these bad plans by using a less straightforward query for the view: SELECT coalesce(contrib.id, cm.contribution_id) AS contribution_id, coalesce(cm.yield, im.yield) AS yield, coalesce(cm.term, im.term) AS term FROM contributions contrib JOIN investment_metrics_view im ON im.investment_id = contrib.investment_id FULL OUTER JOIN contribution_metrics_view cm ON cm.contribution_id = contrib.id Best, ~Alex Reece