在特定联接中忽略了Postgres pg_trgm GIN索引

如何解决在特定联接中忽略了Postgres pg_trgm GIN索引

我有一个表item，其中包含多个文本字段，例如name，unique_attr，category等，而我都使用GIN为所有索引建立了索引（ gin_trgm_ops）索引用于更快的ilike查询，实际上，即使连接到表inventory_membership，索引也可以使用，并可以加快执行时间。我的解释输出：

   explain analyze select i.* from item i 
     join inventory_membership im on im.inventory_id = i.inventory_id
     where i.name ilike '%blu%' or unique_attr ilike '%blu%' or category ilike '%blu%' 
     or brand ilike '%blu%';

Hash Join  (cost=98.64..4584.98 rows=87302 width=478) (actual time=4.258..30.393 rows=57584 loops=1)
  Hash Cond: (i.inventory_id = im.inventory_id)
  ->  Bitmap Heap Scan on item i  (cost=95.45..3584.23 rows=4982 width=478) (actual time=3.706..10.529 rows=3340 loops=1)
        Recheck Cond: ((name ~~* '%blu%'::text) OR (unique_attr ~~* '%blu%'::text) OR (category ~~* '%blu%'::text) OR (brand ~~* '%blu%'::text))
        Heap Blocks: exact=715
        ->  BitmapOr  (cost=95.45..95.45 rows=5130 width=0) (actual time=3.622..3.622 rows=0 loops=1)
              ->  Bitmap Index Scan on item_name_idx  (cost=0.00..42.97 rows=3596 width=0) (actual time=1.612..1.612 rows=3160 loops=1)
                    Index Cond: (name ~~* '%blu%'::text)
              ->  Bitmap Index Scan on item_unique_attr_idx  (cost=0.00..12.01 rows=1 width=0) (actual time=0.586..0.586 rows=32 loops=1)
                    Index Cond: (unique_attr ~~* '%blu%'::text)
              ->  Bitmap Index Scan on item_category_idx  (cost=0.00..22.78 rows=1437 width=0) (actual time=0.888..0.888 rows=1394 loops=1)
                    Index Cond: (category ~~* '%blu%'::text)
              ->  Bitmap Index Scan on item_brand_idx  (cost=0.00..12.72 rows=96 width=0) (actual time=0.532..0.532 rows=42 loops=1)
                    Index Cond: (brand ~~* '%blu%'::text)
  ->  Hash  (cost=1.97..1.97 rows=97 width=4) (actual time=0.059..0.060 rows=87 loops=1)
        Buckets: 1024  Batches: 1  Memory Usage: 12kB
        ->  Seq Scan on inventory_membership im  (cost=0.00..1.97 rows=97 width=4) (actual time=0.010..0.032 rows=87 loops=1)
Planning Time: 0.924 ms
Execution Time: 42.093 ms

我们可以看到item_name_idx，item_unique_attr_idx，item_category_idx和item_brand_idx GIN索引已用于索引条件。很好。

但是，当我联接另一个表（inventory表仅具有id和name列）时，索引消失了。说明：

explain analyze select i.* from item i
    join inventory inv on inv.id = i.inventory_id
    join inventory_membership im on im.inventory_id = i.inventory_id
    where i.name ilike '%blu%' or unique_attr ilike '%blu%' or category ilike '%blu%' or brand 
    ilike '%blu%';

Hash Join  (cost=4.67..1172.61 rows=60407 width=478) (actual time=0.775..121.787 rows=57584 loops=1)
  Hash Cond: (inv.id = im.inventory_id)
  ->  Merge Join  (cost=1.49..440.81 rows=4982 width=482) (actual time=0.111..101.857 rows=3340 loops=1)
        Merge Cond: (i.inventory_id = inv.id)
        ->  Index Scan using item_inventory_id_idx on item i  (cost=0.29..13946.60 rows=4982 width=478) (actual time=0.085..99.857 rows=3340 loops=1)
              Filter: ((name ~~* '%blu%'::text) OR (unique_attr ~~* '%blu%'::text) OR (category ~~* '%blu%'::text) OR (brand ~~* '%blu%'::text))
              Rows Removed by Filter: 34858
        ->  Sort  (cost=1.20..1.22 rows=8 width=4) (actual time=0.020..0.025 rows=8 loops=1)
              Sort Key: inv.id
              Sort Method: quicksort  Memory: 25kB
              ->  Seq Scan on inventory inv  (cost=0.00..1.08 rows=8 width=4) (actual time=0.006..0.009 rows=8 loops=1)
  ->  Hash  (cost=1.97..1.97 rows=97 width=4) (actual time=0.650..0.651 rows=87 loops=1)
        Buckets: 1024  Batches: 1  Memory Usage: 12kB
        ->  Seq Scan on inventory_membership im  (cost=0.00..1.97 rows=97 width=4) (actual time=0.005..0.028 rows=87 loops=1)
Planning Time: 7.193 ms
Execution Time: 132.427 ms

您会看到GIN索引不见了，解释使用的唯一索引是item_inventory_id_idx-这是常规的FK BTREE索引。而且，执行时间花了很多时间。为什么？

解决方法

您注意到您对库存名称最感兴趣，并且库存表中只有8行。 8行是查询计划者偏爱merge join而不是hash join的原因，当两个表都很大时，它会更好。合并联接需要将inventory_id放在排序列表中（这恰好是索引），这意味着它不希望使用GIN索引，因为它认为效率较低。

现在，没有数据了，您可以做几件事，但我不知道哪种会更快。您已经尝试过的第一个方法是在scalar subquery中获取广告资源名称：

SELECT i.*,(select name from inventory where id = i.inventory_id) as inventoryName
FROM item i
JOIN inventory_membership im ON im.inventory_id = i.inventory_id
WHERE i.name ilike '%blu%' or unique_attr ilike '%blu%' or category ilike '%blu%' 
     or brand ilike '%blu%';

但这意味着此select语句执行了57k次，每行执行一次。第二种是使用您已有的查询，但要查看在i.inventory_id中将inv.id更改为inventory_membership是否有任何改变。

SELECT i.*,inv.name as inventoryName
FROM item i
JOIN inventory inv ON inv.id = i.inventory_id
JOIN inventory_membership im ON im.inventory_id = inv.id -- <- this changed
WHERE i.name ilike '%blu%' or unique_attr ilike '%blu%' or category ilike '%blu%' 
     or brand ilike '%blu%';

最后，就像在this问题中所说的那样，您可以使用CTE或带有OFFSET 0的子查询来强制执行第一个查询，然后获取清单名称。

WITH my_items AS (
  SELECT i.*
  FROM item i
  JOIN inventory_membership im ON im.inventory_id = i.inventory_id
  WHERE i.name ilike '%blu%' or unique_attr ilike '%blu%' or category ilike '%blu%' 
       or brand ilike '%blu%'
)
SELECT i.*,inv.name as inventoryName
FROM my_items i
JOIN inventory inv ON inv.id = i.inventory_id

或

SELECT i.*,inv.name as inventoryName
FROM (
  SELECT i.*
  FROM item i
  JOIN inventory_membership im ON im.inventory_id = i.inventory_id
  WHERE i.name ilike '%blu%' or unique_attr ilike '%blu%' or category ilike '%blu%' 
       or brand ilike '%blu%'
  OFFSET 0 -- <- this forces the subquery to be evaluated separate from the rest of the query
) i
JOIN inventory inv ON inv.id = i.inventory_id

在特定联接中忽略了Postgres pg_trgm GIN索引

如何解决在特定联接中忽略了Postgres pg_trgm GIN索引

解决方法

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