我们如何优化以下查询：
select *
from program_infos pi
join endeavour_organizations seller_organization on seller_organization.id = pi.supplier_id
join endeavour_organizations obligor_organization on obligor_organization.id = pi.buyer_id
join invoices i on pi.program_id = i.program_id
join assets fa on fa.invoice_id = i.id and fa.owner_id=pi.fi_id
join assets sa on sa.invoice_id = i.id and sa.owner_id=pi.supplier_id;
相应的Explain Analyze是：
Nested Loop (cost=36.94..70919.65 rows=505 width=793) (actual time=0.263..1729.519 rows=267238 loops=1)
-> Nested Loop (cost=36.79..70806.58 rows=505 width=718) (actual time=0.261..1405.417 rows=267238 loops=1)
Join Filter: ((i.id = fa.invoice_id) AND (pi.fi_id = fa.owner_id))
Rows Removed by Join Filter: 400287
-> Hash Join (cost=36.37..69201.99 rows=2567 width=626) (actual time=0.255..772.895 rows=248735 loops=1)
Hash Cond: (pi.supplier_id = seller_organization.id)
-> Hash Join (cost=27.52..68973.45 rows=15977 width=551) (actual time=0.202..672.442 rows=248735 loops=1)
Hash Cond: ((sa.owner_id = pi.supplier_id) AND (i.program_id = pi.program_id))
-> Merge Join (cost=1.29..63781.02 rows=667525 width=288) (actual time=0.021..496.274 rows=667525 loops=1)
Merge Cond: (i.id = sa.invoice_id)
-> Index Scan using invoices_pkey on invoices i (cost=0.42..27363.52 rows=249447 width=196) (actual time=0.004..60.598 rows=249440 loops=1)
-> Index Scan using index_assets_invoice on assets sa (cost=0.42..27450.72 rows=667525 width=92) (actual time=0.014..147.276 rows=667525 loops=1)
-> Hash (cost=20.09..20.09 rows=409 width=263) (actual time=0.176..0.176 rows=409 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 133kB
-> Seq Scan on program_infos pi (cost=0.00..20.09 rows=409 width=263) (actual time=0.001..0.064 rows=409 loops=1)
-> Hash (cost=5.60..5.60 rows=260 width=75) (actual time=0.049..0.049 rows=260 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 27kB
-> Seq Scan on endeavour_organizations seller_organization (cost=0.00..5.60 rows=260 width=75) (actual time=0.006..0.019 rows=260 loops=1)
-> Index Scan using index_assets_owner_invoice on assets fa (cost=0.42..0.57 rows=4 width=92) (actual time=0.001..0.002 rows=3 loops=248735)
Index Cond: (invoice_id = sa.invoice_id)
-> Index Scan using endeavour_organizations_pkey on endeavour_organizations obligor_organization (cost=0.15..0.21 rows=1 width=75) (actual time=0.001..0.001 rows=1 loops=267238)
Index Cond: (id = pi.buyer_id)
Planning time: 3.194 ms
Execution time: 1740.875 ms
(24 rows)
索引位于pi.program_id，pi.fi_id，pi.supplier_id，asset.invoice_id，i.program_id
我无法理解为什么它会做嵌套循环。如果需要其他东西，请告诉我。