Postgresql - 執行DDL - 鎖問題的避免

1. 加列，減列很快，幾乎沒影響

Version 10
alter table test01 add column col11 varchar(100)
alter table test01 alter COLUMN col12 set default 'abc';
Version 11
alter table test01 add column col11 varchar(100) default 'abc';
alter table test01 add column col12 varchar(100) not null default 'abc';
alter table test01 drop column col12 ;

2. 改列的數據類型

   1. 加新列
   2. 同時寫（trigger）
   3. copy old column values to new column.
   4. rename column
   5. drop old column

3. 索引（新建，刪除）

create index concurrently on test01(col1);
drop index CONCURRENTLY test01_id_idx ;
索引重建
使用concurrently命令建新並刪舊，不要直接使用reindex！

4. constraints
   先not valid，再valid，時間短。

ALTER TABLE <table> ADD CONSTRAINT <name> CHECK (<column> IS NOT NULL) NOT VALID;
Manually verify that all rows have non-null values in your column.
ALTER TABLE <table> VALIDATE CONSTRAINT <name>;
alter table test02 add foreign key (id) references test01(id) not valid;
alter table test02 validate constraint test02_id_fkey ;
ALTER TABLE ... ADD CONSTRAINT ... CHECK (...) NOT VALID: 
ALTER TABLE ... VALIDATE CONSTRAINT <constraint>: 

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常用腳本

1. 查看鎖

SELECT
  psa.datname as database,
  psa.query as current_query,
  clock_timestamp() - psa.xact_start AS transaction_age,
  array_agg(distinct c.relname) AS tables_with_locks
FROM pg_catalog.pg_stat_activity psa
JOIN pg_catalog.pg_locks l ON (psa.pid = l.pid)
JOIN pg_catalog.pg_class c ON (l.relation = c.oid)
JOIN pg_catalog.pg_namespace ns ON (c.relnamespace = ns.oid)
WHERE psa.pid != pg_backend_pid()
  AND ns.nspname != 'pg_catalog'
  AND c.relkind = 'r'
  AND psa.xact_start < clock_timestamp() - '5 seconds'::interval
GROUP BY psa.datname, psa.query, psa.xact_start;

2. 查看錶統計信息

SELECT
  seq_scan,
  seq_tup_read,
  idx_scan,
  idx_tup_fetch,
  n_tup_ins,
  n_tup_upd,
  n_tup_del
FROM pg_catalog.pg_stat_user_tables
WHERE relname = '<table>';

3. 查看對象

SELECT
  relname,
  relfilenode
FROM pg_catalog.pg_class
WHERE relname in (
  '<table>',
  '<index>'
)
-- Order by oid for convenience if you're checking multiple relations.
ORDER BY oid;

SELECT
  d.objid::regclass AS owning_object,
  d.refobjid::regclass AS dependent_object,
  a.attname AS dependent_column,
  d.deptype -- The meaning of this type is documented at https://www.postgresql.org/docs/current/catalog-pg-depend.html
FROM pg_catalog.pg_depend d
LEFT JOIN pg_catalog.pg_attribute a ON d.refobjid = a.attrelid
  AND d.refobjsubid = a.attnum
WHERE refobjid = '<table>'::regclass
  AND a.attname = '<column>';