一般來說，打了 index 都會想看成效，使用的情況如何，在 postgreSQL 內可以直接下這行指令
SELECT * FROM pg_stat_user_indexes;
就可以拿到完整的資料去分析每個 index 的使用情況。
SELECT * FROM pg_stat_user_indexes;
relid | indexrelid | schemaname | relname | indexrelname | idx_scan | idx_tup_read | idx_tup_fetch
-------+------------+------------+------------+-------------------------+----------+--------------+---------------
16454 | 16491 | public | categories | categories_pkey | 0 | 0 | 0
16463 | 16493 | public | customers | customers_pkey | 0 | 0 | 0
16470 | 16495 | public | inventory | inventory_pkey | 0 | 0 | 0
16478 | 16497 | public | orders | orders_pkey | 0 | 0 | 0
16484 | 16499 | public | products | products_pkey | 0 | 0 | 0
16458 | 16501 | public | cust_hist | ix_cust_hist_customerid | 0 | 0 | 0
16463 | 16502 | public | customers | ix_cust_username | 0 | 0 | 0
16478 | 16503 | public | orders | ix_order_custid | 0 | 0 | 0
16473 | 16504 | public | orderlines | ix_orderlines_orderid | 0 | 0 | 0
16484 | 16505 | public | products | ix_prod_category | 0 | 0 | 0
16484 | 16506 | public | products | ix_prod_special | 0 | 0 | 0
(11 rows)
值得注意的是，有些數據為 0 並不代表真的沒用到，因為
- unique 也是一種 index
- primary key 也是一種 index