Postgresql之amcheck验证索引完整性

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Postgresql10版本开始提供了该插件验证索引或者是表的逻辑一致性。比如系统升级后，collate和原来不一致，数据库里索引的顺序和该collate不匹配。物理磁盘损坏，文件系统损坏,内存故障等，导致数据库中的索引也损坏。

可以通过该插件进行验证检测，注意该插件只验证，不会修补。如果通过该插件找出相应的索引，可以使用reindex命令重建索引，如果数据库很小，也可以考虑使用reindexdb重建整个库的索引。

另外从PG14版本开始新增了verify_heapam，可以验证表的数据页。
相关函数：

bt_index_check   --加accessshared，和select类似
bt_index_parent_check   --加ShareLock锁，和DML均会冲突，所以使用该函数要注意

使用例子：
bt_index_check

test=# SELECT bt_index_check(index => c.oid, heapallindexed => i.indisunique),
               c.relname,
               c.relpages
FROM pg_index i
JOIN pg_opclass op ON i.indclass[0] = op.oid
JOIN pg_am am ON op.opcmethod = am.oid
JOIN pg_class c ON i.indexrelid = c.oid
JOIN pg_namespace n ON c.relnamespace = n.oid
WHERE am.amname = 'btree' AND n.nspname = 'pg_catalog'
-- Don't check temp tables, which may be from another session:
AND c.relpersistence != 't'
-- Function may throw an error when this is omitted:
AND c.relkind = 'i' AND i.indisready AND i.indisvalid
ORDER BY c.relpages DESC LIMIT 10;
 bt_index_check |             relname             | relpages
----------------+---------------------------------+----------
                | pg_depend_reference_index       |       43
                | pg_depend_depender_index        |       40
                | pg_proc_proname_args_nsp_index  |       31
                | pg_description_o_c_o_index      |       21
                | pg_attribute_relid_attnam_index |       14
                | pg_proc_oid_index               |       10
                | pg_attribute_relid_attnum_index |        9
                | pg_amproc_fam_proc_index        |        5
                | pg_amop_opr_fam_index           |        5
                | pg_amop_fam_strat_index         |        5
(10 rows)

bt_index_parent_check

--验证tbl表的b-tree索引
create extension if not exists amcheck;
set statement_timeout to 0;
do $$
declare
  r record;
  sql text;
  ts_pre timestamptz;
  e_message text;
  e_detail text;
  e_context text;
  e_hint text;
  errcount int := 0;
begin
  raise info 'begin!...';
  for r in
    select
      row_number() over(order by tc.reltuples) as i,
      count(*) over() as cnt,
      c.oid,
      i.indisunique,
      c.relname,
      c.relpages::int8,
      tc.reltuples::int8 as tuples
    from pg_index i
    join pg_opclass op on i.indclass[0] = op.oid
    join pg_am am on op.opcmethod = am.oid
    join pg_class c on i.indexrelid = c.oid
    join pg_class tc on i.indrelid = tc.oid
    join pg_namespace n on c.relnamespace = n.oid
    where
      am.amname = 'btree'
      --and n.nspname = 'public'
      and c.relpersistence <> 't' -- don't check temp tables
      and c.relkind = 'i'
      and i.indisready
      and i.indisvalid
      and tc.relname = 'tbl' -- comment this out to check the whole DB
      --and c.relname in ('index_projects_on_name_and_id', 'index_projects_on_lower_name', 'index_projects_api_name_id_desc') 
    order by tc.reltuples
  loop
    ts_pre := clock_timestamp();
    raise info '[%] Processing %/%: index: % (index relpages: %; heap tuples: ~%)...',
      ts_pre::timestamptz(3), r.i, r.cnt, r.relname, r.relpages, r.tuples;
    begin
      perform bt_index_parent_check(index => r.oid, heapallindexed => true);
      raise info '[%] SUCCESS %/% – index: %. Time taken: %',
        clock_timestamp()::timestamptz(3), r.i, r.cnt, r.relname, (clock_timestamp() - ts_pre);
    exception when others then
      get stacked diagnostics
        e_message = message_text,
        e_detail = pg_exception_detail,
        e_context = pg_exception_context,
        e_hint = pg_exception_hint;
      errcount := errcount + 1;
      raise warning $err$[%] FAILED %/% – index: %.
ERROR: %
CONTEXT: %
DETAIL: %
HINT: %
$err$,
        clock_timestamp()::timestamptz(3), r.i, r.cnt, r.relname, e_message, e_detail, e_context, e_hint;
    end;
  end loop;
  if errcount = 0 then
    raise info 'Btree index scan with amcheck successfully finished. 0 errors.';
  else
    raise exception 'Index corruption detected by amcheck, % errors, see details in the log.', errcount;
  end if;
end $$;

INFO:  begin!...
INFO:  [2021-12-20 14:52:14.679+08] Processing 1/1: index: tbl_pkey (index relpages: 552; heap tuples: ~200300)...
INFO:  [2021-12-20 14:52:14.681+08] SUCCESS 1/1 – index: tbl_pkey. Time taken: 00:00:00.002516
INFO:  Btree index scan with amcheck successfully finished. 0 errors.
DO

参考：
https://www.postgresql.org/docs/current/amcheck.html
https://elephanttamer.net/?p=61

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