本節簡單介紹了PostgreSQL中用於索引維護的一些查詢.

**檢視錶&索引大小**
```
SELECT    CONCAT(n.nspname,'.', c.relname) AS table,
          i.relname AS index_name, pg_size_pretty(pg_relation_size(x.indrelid)) AS table_size,
          pg_size_pretty(pg_relation_size(x.indexrelid)) AS index_size,
          pg_size_pretty(pg_total_relation_size(x.indrelid)) AS total_size FROM pg_class c 
JOIN      pg_index x ON c.oid = x.indrelid
JOIN      pg_class i ON i.oid = x.indexrelid
LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE     c.relkind = ANY (ARRAY['r', 't'])
AND       n.oid NOT IN (99, 11, 12375);

[local:/data/run/pg12]:5120 pg12@testdb=# SELECT    CONCAT(n.nspname,'.', c.relname) AS table,
pg12@testdb-#           i.relname AS index_name, pg_size_pretty(pg_relation_size(x.indrelid)) AS table_size,
pg12@testdb-#           pg_size_pretty(pg_relation_size(x.indexrelid)) AS index_size,
pg12@testdb-#           pg_size_pretty(pg_total_relation_size(x.indrelid)) AS total_size FROM pg_class c 
pg12@testdb-# JOIN      pg_index x ON c.oid = x.indrelid
pg12@testdb-# JOIN      pg_class i ON i.oid = x.indexrelid
pg12@testdb-# LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
pg12@testdb-# WHERE     c.relkind = ANY (ARRAY['r', 't'])
pg12@testdb-# AND       n.oid NOT IN (99, 11, 12375);
      table       |    index_name    | table_size | index_size | total_size 
------------------+------------------+------------+------------+------------
 public.test      | test_pkey        | 0 bytes    | 8192 bytes | 16 kB
 public.t_pgbench | idx_t_pgbench_c1 | 425 MB     | 214 MB     | 639 MB
 public.tbl1      | tbl1_pkey        | 5096 kB    | 2208 kB    | 7312 kB
(3 rows)

```

**索引定義**
```
SELECT pg_get_indexdef(indexrelid) AS index_query
FROM   pg_index WHERE  indrelid = 'test'::regclass;

[local:/data/run/pg12]:5120 pg12@testdb=# SELECT pg_get_indexdef(indexrelid) AS index_query
FROM   pg_index WHERE  indrelid = 'test'::regclass;
                          index_query                          
---------------------------------------------------------------
 CREATE UNIQUE INDEX test_pkey ON public.test USING btree (id)
(1 row)

```

**識別未使用的Index**
```
SELECT s.relname AS table_name,
       indexrelname AS index_name,
       i.indisunique,
       idx_scan AS index_scans
FROM   pg_catalog.pg_stat_user_indexes s,
       pg_index i
WHERE  i.indexrelid = s.indexrelid;

```

**檢索重複的索引**
```
SELECT   indrelid::regclass table_name,
         att.attname column_name,
         amname index_method
FROM     pg_index i,
         pg_class c,
         pg_opclass o,
         pg_am a,
         pg_attribute att
WHERE    o.oid = ALL (indclass) 
AND      att.attnum = ANY(i.indkey)
AND      a.oid = o.opcmethod
AND      att.attrelid = c.oid
AND      c.oid = i.indrelid
GROUP BY table_name, 
         att.attname,
         indclass,
         amname, indkey
HAVING count(*) > 1;

[local:/data/run/pg12]:5120 pg12@testdb=# CREATE UNIQUE INDEX test_pkey_dup ON public.test USING btree (id);
CREATE INDEX
[local:/data/run/pg12]:5120 pg12@testdb=# SELECT   indrelid::regclass table_name,
pg12@testdb-#          att.attname column_name,
pg12@testdb-#          amname index_method
pg12@testdb-# FROM     pg_index i,
pg12@testdb-#          pg_class c,
pg12@testdb-#          pg_opclass o,
pg12@testdb-#          pg_am a,
pg12@testdb-#          pg_attribute att
pg12@testdb-# WHERE    o.oid = ALL (indclass) 
pg12@testdb-# AND      att.attnum = ANY(i.indkey)
pg12@testdb-# AND      a.oid = o.opcmethod
pg12@testdb-# AND      att.attrelid = c.oid
pg12@testdb-# AND      c.oid = i.indrelid
pg12@testdb-# GROUP BY table_name, 
pg12@testdb-#          att.attname,
pg12@testdb-#          indclass,
pg12@testdb-#          amname, indkey
pg12@testdb-# HAVING count(*) > 1;
 table_name | column_name | index_method 
------------+-------------+--------------
 test       | id          | btree
(1 row)

```

**參考資料**
[Useful Queries For PostgreSQL Index Maintenance](https://www.percona.com/blog/2020/03/31/useful-queries-for-postgresql-index-maintenance/)

PostgreSQL DBA(170) - pgAdmin(Queries For Index Maintenance)

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