在PostgreSQL統計中，idx_scan非常低

Question

我想問PostgresSQL優化中的幫助。

我有一箇中等大小的表（約2000000條記錄），而且我寫了一個非常簡單的查詢，如：

SELECT COUNT (DISTINCT(user_id)) 
FROM fp left outer join sd ON fp.session_id = sd.session_id 
WHERE fp.license_key = 'license' AND sd.device_hash = 'hash' 

我對「許可密鑰」，「USER_ID」和「device_hash」簡單指標（3個索引） 執行程序不想使用我的license_key索引，因爲表中有超過700.000個匹配，Seq Scan是更好的選擇。

Aggregate (cost=396247.61..396247.62 rows=1 width=17) 

    -> Hash Join (cost=99668.54..396152.13 rows=38195 width=17) 

     Hash Cond: ((fp.session_id)::text = (sd.session_id)::text) 

     -> Seq Scan on fp (cost=0.00..293450.55 rows=706957 width=45) 

       Filter: ((license_key)::text = 'license'::text) 

     -> Hash (cost=98678.10..98678.10 rows=79235 width=28) 

       -> Bitmap Heap Scan on sd (cost=2902.50..98678.10 rows=79235 width=28) 

        Recheck Cond: ((device_hash)::text = 'hash'::text) 

        -> Bitmap Index Scan on "sd.device_hash_btree_idx" (cost=0.00..2882.69 rows=79235 width=0) 

          Index Cond: ((device_hash)::text = 'hash'::text) 

我檢查了我的統計數據：

select * from where indexrelname= 'fp_license_key_btree_idx' 

relid | indexrelname | idx_scan | idx_tup_read | idx_fetch 
---------------------------------------------------------------------------- 
16430 |fp.license_key_btree_idx | 451 | 13641445  | 13641445 

你能不能給我建議？我如何改進我的idx_scan？謝謝

Answer 1

你可以嘗試用CTE和新指數：

CREATE INDEX ON fp USING btree(session_id); --maybe cover license_key as well, but doubt it 

WITH s AS (SELECT session_id FROM sd WHERE sd.device_hash = 'hash'), 
    f AS (SELECT user_id, session_id 
      FROM fp 
      WHERE fp.license_key = 'license' 
       AND session_id = ANY((SELECT array_agg(session_id) FROM s)::int[])) 
SELECT COUNT (DISTINCT(user_id)) 
FROM f 
JOIN s ON f.session_id = s.session_id; --your left join is not left join anyway