我有两个问题：
SELECT "recipes_recipe"."short_name",
COUNT(DISTINCT "recipes_recipe"."quantity_type") AS "quantity_type_count",
SUM("measures_measure"."standard") AS "volume",
CASE WHEN COUNT(DISTINCT "recipes_recipe"."quantity_type") = 1
THEN (SUM((T7."standard" * T8."standard")) / SUM(T8."standard"))
ELSE NULL END AS "weighted_temperature"
FROM "orders_orderitemresult"
INNER JOIN "orders_orderitem"
ON ( "orders_orderitemresult"."order_line_id" = "orders_orderitem"."id" )
INNER JOIN "orders_order"
ON ( "orders_orderitem"."order_id" = "orders_order"."id" )
INNER JOIN "recipes_recipe"
ON ( "orders_orderitem"."recipe_id" = "recipes_recipe"."id" )
LEFT OUTER JOIN "measures_measure"
ON ( "orders_orderitemresult"."measured_volume_id" = "measures_measure"."id" )
LEFT OUTER JOIN "measures_measure" T7
ON ( "orders_orderitemresult"."temperature_id" = T7."id" )
INNER JOIN "measures_measure" T8
ON ( "orders_orderitemresult"."loaded_quantity_id" = T8."id" )
WHERE ("orders_orderitemresult"."deleted" = False
AND "orders_order"."freight_id" IN (
SELECT U0."id"
FROM "freights_freight" U0
WHERE (
U0."deleted" = False
AND U0."state" = 'completed'
AND U0."completed_date" > '2015-06-06 00:00:00+00:00'
) ) )
GROUP BY "recipes_recipe"."short_name"
ORDER BY "recipes_recipe"."short_name" ASC
另外还有一个JOIN：
SELECT "recipes_recipe"."short_name",
COUNT(DISTINCT "recipes_recipe"."quantity_type") AS "quantity_type_count",
SUM("measures_measure"."standard") AS "volume",
CASE WHEN COUNT(DISTINCT "recipes_recipe"."quantity_type") = 1
THEN (SUM((T7."standard" * T8."standard")) / SUM(T8."standard"))
ELSE NULL END AS "weighted_temperature"
FROM "orders_orderitemresult"
INNER JOIN "orders_orderitem"
ON ( "orders_orderitemresult"."order_line_id" = "orders_orderitem"."id" )
INNER JOIN "orders_order"
ON ( "orders_orderitem"."order_id" = "orders_order"."id" )
INNER JOIN "recipes_recipe"
ON ( "orders_orderitem"."recipe_id" = "recipes_recipe"."id" )
LEFT OUTER JOIN "measures_measure"
ON ( "orders_orderitemresult"."measured_volume_id" = "measures_measure"."id" )
LEFT OUTER JOIN "measures_measure" T7
ON ( "orders_orderitemresult"."temperature_id" = T7."id" )
INNER JOIN "measures_measure" T8
ON ( "orders_orderitemresult"."loaded_quantity_id" = T8."id" )
LEFT OUTER JOIN "measures_measure" T9
ON ( "orders_orderitemresult"."converted_volume_id" = T9."id" )
WHERE ("orders_orderitemresult"."deleted" = False
AND "orders_order"."freight_id" IN (
SELECT U0."id"
FROM "freights_freight" U0
WHERE (
U0."deleted" = False
AND U0."state" = 'completed'
AND U0."completed_date" > '2015-06-06 00:00:00+00:00'
)))
GROUP BY "recipes_recipe"."short_name"
ORDER BY "recipes_recipe"."short_name" ASC
这些查询由Django ORM生成。第一个需要3ms而第二个需要120s（！）。我做了VACUUM ANALYZE。 EXPLAIN ANALYZE给出以下内容：
什么可以导致查询规划器这样做？为什么索引没有在第二个查询中使用？
我尝试禁用哈希，但它使用了更复杂的方法。
编辑：
有关索引的更多信息：
table_name | index_name | column_names
------------------------+-------------------------------------------------------------+----------------------------------
measures_measure | measures_measure_pkey | id
orders_orderitem | orders_orderitem_69dfcb07 | order_id
orders_orderitem | orders_orderitem_7339046a | completed_date
orders_orderitem | orders_orderitem_9ed39e2e | state
orders_orderitem | orders_orderitem_da50e9c3 | recipe_id
orders_orderitem | orders_orderitem_pkey | id
orders_orderitem | orders_orderitem_quantity_id_key | quantity_id
orders_orderitem | orders_orderitem_state_68c8c3135683908e_like | state
orders_orderitemresult | orders_orderitemresult_5fc62ccf | order_line_id
orders_orderitemresult | orders_orderitemresult_8424d087 | creation_date
orders_orderitemresult | orders_orderitemresult_98ea6344 | result_type
orders_orderitemresult | orders_orderitemresult_c00077c4 | loading_rack_id
orders_orderitemresult | orders_orderitemresult_converted_volume_id_key | converted_volume_id
orders_orderitemresult | orders_orderitemresult_counter_end_id_key | counter_end_id
orders_orderitemresult | orders_orderitemresult_counter_start_id_key | counter_start_id
orders_orderitemresult | orders_orderitemresult_e35d4212 | loading_arm_id
orders_orderitemresult | orders_orderitemresult_loaded_quantity_id_key | loaded_quantity_id
orders_orderitemresult | orders_orderitemresult_mass_id_key | mass_id
orders_orderitemresult | orders_orderitemresult_measure_temperature_density_id_key | measure_temperature_density_id
orders_orderitemresult | orders_orderitemresult_measured_volume_id_key | measured_volume_id
orders_orderitemresult | orders_orderitemresult_pkey | id
orders_orderitemresult | orders_orderitemresult_reference_temperature_density_id_key | reference_temperature_density_id
orders_orderitemresult | orders_orderitemresult_result_type_19bfde1efbd2fb2e_like | result_type
orders_orderitemresult | orders_orderitemresult_target_quantity_id_key | target_quantity_id
orders_orderitemresult | orders_orderitemresult_temperature_id_key | temperature_id
recipes_recipe | recipes_recipe_9bea82de | product_id
recipes_recipe | recipes_recipe_bdc611c2 | external_code
recipes_recipe | recipes_recipe_c76a5e84 | active
recipes_recipe | recipes_recipe_code_64a829b99ae673f2_like | code
recipes_recipe | recipes_recipe_code_key | code
recipes_recipe | recipes_recipe_external_code_492873921b11a1bf_like | external_code
recipes_recipe | recipes_recipe_long_name_38b0490ebf585c5c_like | long_name
recipes_recipe | recipes_recipe_long_name_key | long_name
recipes_recipe | recipes_recipe_pkey | id
recipes_recipe | recipes_recipe_short_name_5e9f02ca3b6b7843_like | short_name
recipes_recipe | recipes_recipe_short_name_key | short_name
答案 0 :(得分：2)
使用子选择来限制笛卡尔积的范围，这将大大提高您的查询成本。
像
这样的东西SELECT "recipes_recipe"."short_name",
COUNT(DISTINCT "recipes_recipe"."quantity_type") AS "quantity_type_count",
SUM("measures_measure"."standard") AS "volume",
CASE WHEN COUNT(DISTINCT "recipes_recipe"."quantity_type") = 1
THEN (SUM((T7."standard" * T8."standard")) / SUM(T8."standard"))
ELSE NULL END AS "weighted_temperature",
SUM(T9."standard") AS "converted_volume"
FROM (
SELECT
orders_orderitem.recipe_id,
orders_orderitem.id as orders_orderitem_id,
"orders_orderitemresult"."measured_volume_id",
"orders_orderitemresult"."temperature_id",
"orders_orderitemresult"."loaded_quantity_id",
"orders_orderitemresult"."converted_volume_id"
FROM "orders_orderitemresult"
INNER JOIN "orders_orderitem"
ON ( "orders_orderitemresult"."order_line_id" = "orders_orderitem"."id" )
INNER JOIN "orders_order"
ON ( "orders_orderitem"."order_id" = "orders_order"."id" )
WHERE (
"orders_orderitemresult"."deleted" = False
AND "orders_order"."freight_id" IN (
SELECT U0."id"
FROM "freights_freight" U0
WHERE (
U0."deleted" = False
AND U0."state" = 'completed'
AND U0."completed_date" > '2015-06-06 00:00:00+00:00'
)))) subselect
INNER JOIN "recipes_recipe"
ON ( "subselect"."recipe_id" = "recipes_recipe"."id" )
INNER JOIN "measures_measure"
ON ( "subselect"."measured_volume_id" = "measures_measure"."id" )
INNER JOIN "measures_measure" T7
ON ( "subselect"."temperature_id" = T7."id" )
INNER JOIN "measures_measure" T8
ON ( "subselect"."loaded_quantity_id" = T8."id" )
INNER JOIN "measures_measure" T9
ON ( "subselect"."converted_volume_id" = T9."id" )
GROUP BY "recipes_recipe"."short_name"
ORDER BY "recipes_recipe"."short_name" ASC;
应该更好