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這篇文章主要介紹“PostgreSQL中的Multi Version Heap Tuple分析”，在日常操作中，相信很多人在PostgreSQL中的Multi Version Heap Tuple分析問題上存在疑惑，小編查閱了各式資料，整理出簡單好用的操作方法，希望對大家解答”PostgreSQL中的Multi Version Heap Tuple分析”的疑惑有所幫助！接下來，請跟著小編一起來學習吧！
Concurrency Control并發控制是一種機制，在并發進行多個事務時維護一致性(Consistency)和隔離性(Isolation)，一致性和隔離性是數據庫事務ACID(Atomicity, Consistency, Isolation, Durability) 屬性中的C和I。
多版本并發控制(MVCC)是廣泛使用的并發控制技術,其主要優勢是讀不會阻塞寫，而寫也不會阻塞讀。MVCC有很多種變體，PostgreSQL使用一種稱為快照隔離Snapshot Isolation (SI)的MVCC變體實現并發控制。
在MVCC中，每個DML操作創建一個數據（包括Index）的新版本，同時保留之前的舊版本。當事務讀取數據時，選擇其中一個“正確”的版本，以確保各個事務之間的隔離。
為了更好的說明Heap Tuple的存儲結構,有必要先簡要說明Tuple的隱藏列以及相關的標記.
隱藏列
testdb=# select attname, attnum, atttypid::regtype, attisdropped::text from pg_attribute where attrelid=34374; attname | attnum | atttypid | attisdropped ----------+--------+-------------------+-------------- tableoid | -7 | oid | false cmax | -6 | cid | false xmax | -5 | xid | false cmin | -4 | cid | false xmin | -3 | xid | false ctid | -1 | tid | false c1 | 1 | integer | false c2 | 2 | character varying | false c3 | 3 | character varying | false (9 rows)
tableoid-數據表OID
cmax-刪除該tuple的事務內部命令ID
xmax-刪除該tuple的事務ID
cmin-插入該tuple的事務內部命令ID
xmin-插入該tuple的事務ID
ctid-heap tuple的ID
infomask標記
主要的標記包括t_infomask2和t_infomask.
t_infomask2
取值和說明如下
/* * information stored in t_infomask2: */ #define HEAP_NATTS_MASK 0x07FF /* 11 bits for number of attributes */ //低11位為屬性個數 /* bits 0x1800 are available */ #define HEAP_KEYS_UPDATED 0x2000 /* tuple was updated and key cols * modified, or tuple deleted */ #define HEAP_HOT_UPDATED 0x4000 /* tuple was HOT-updated */ #define HEAP_ONLY_TUPLE 0x8000 /* this is heap-only tuple */ #define HEAP2_XACT_MASK 0xE000 /* visibility-related bits */ //把十六進制值轉換為二進制顯示 11111111111 #define HEAP_NATTS_MASK 0x07FF 10000000000000 #define HEAP_KEYS_UPDATED 0x2000 100000000000000 #define HEAP_HOT_UPDATED 0x4000 1000000000000000 #define HEAP_ONLY_TUPLE 0x8000 1110000000000000 #define HEAP2_XACT_MASK 0xE000 1111111111111110 #define SpecTokenOffsetNumber 0xfffe
t_infomask
取值和說明如下
//t_infomask說明 1 #define HEAP_HASNULL 0x0001 /* has null attribute(s) */ 10 #define HEAP_HASVARWIDTH 0x0002 /* has variable-width attribute(s) */ 100 #define HEAP_HASEXTERNAL 0x0004 /* has external stored attribute(s) */ 1000 #define HEAP_HASOID 0x0008 /* has an object-id field */ 10000 #define HEAP_XMAX_KEYSHR_LOCK 0x0010 /* xmax is a key-shared locker */ 100000 #define HEAP_COMBOCID 0x0020 /* t_cid is a combo cid */ 1000000 #define HEAP_XMAX_EXCL_LOCK 0x0040 /* xmax is exclusive locker */ 10000000 #define HEAP_XMAX_LOCK_ONLY 0x0080 /* xmax, if valid, is only a locker */ /* xmax is a shared locker */ #define HEAP_XMAX_SHR_LOCK (HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_KEYSHR_LOCK) #define HEAP_LOCK_MASK (HEAP_XMAX_SHR_LOCK | HEAP_XMAX_EXCL_LOCK | \ HEAP_XMAX_KEYSHR_LOCK) 100000000 #define HEAP_XMIN_COMMITTED 0x0100 /* t_xmin committed */ 1000000000 #define HEAP_XMIN_INVALID 0x0200 /* t_xmin invalid/aborted */ #define HEAP_XMIN_FROZEN (HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID) 10000000000 #define HEAP_XMAX_COMMITTED 0x0400 /* t_xmax committed */ 100000000000 #define HEAP_XMAX_INVALID 0x0800 /* t_xmax invalid/aborted */ 1000000000000 #define HEAP_XMAX_IS_MULTI 0x1000 /* t_xmax is a MultiXactId */ 10000000000000 #define HEAP_UPDATED 0x2000 /* this is UPDATEd version of row */ 100000000000000 #define HEAP_MOVED_OFF 0x4000 /* moved to another place by pre-9.0 * VACUUM FULL; kept for binary * upgrade support */ 1000000000000000 #define HEAP_MOVED_IN 0x8000 /* moved from another place by pre-9.0 * VACUUM FULL; kept for binary * upgrade support */ #define HEAP_MOVED (HEAP_MOVED_OFF | HEAP_MOVED_IN) 1111111111110000 #define HEAP_XACT_MASK 0xFFF0 /* visibility-related bits */
創建數據表,插入數據
testdb=# drop table if exists t_mvcc1; DROP TABLE testdb=# create table t_mvcc1 (c1 int,c2 varchar(40)); CREATE TABLE testdb=# testdb=# insert into t_mvcc1 values(1,'C2-1'); INSERT 0 1 testdb=# insert into t_mvcc1 values(2,'C2-2'); INSERT 0 1 testdb=#
通過pageinspect插件查看page中的內容
testdb=# select lp,lp_off,lp_flags,t_xmin,t_xmax,t_field3 as t_cid,t_ctid,t_infomask2,t_infomask from heap_page_items(get_raw_page('t_mvcc1',0));
lp | lp_off | lp_flags | t_xmin | t_xmax | t_cid | t_ctid | t_infomask2 | t_infomask
----+--------+----------+--------+--------+-------+--------+-------------+------------
1 | 8152 | 1 | 2300 | 0 | 0 | (0,1) | 2 | 2050
2 | 8112 | 1 | 2301 | 0 | 0 | (0,2) | 2 | 2050
(2 rows)其中lp為Line Pointer(ItemID,行指針),t_xmin(分別是2300&2301)為插入數據的事務ID,t_xmax為0(Invalid事務號),t_cid是命令編號,t_ctid是heap tuple ID,詳細解釋請參見參考資料.
t_infomask2為0x0002,說明有2個字段(低11位為屬性的個數);
t_infomask為2050,即0x0802,標記存在可變長屬性（HEAP_HASVARWIDTH）/XMAX無效（HEAP_XMAX_INVALID）
更新數據(提交事務)
testdb=# testdb=# begin; BEGIN testdb=# testdb=# update t_mvcc1 set c2='C2#1' where c1 = 1; UPDATE 1 testdb=# update t_mvcc1 set c2='C2#2' where c1 = 2; UPDATE 1 testdb=# testdb=# commit; COMMIT
通過pageinspect插件查看page中的內容
testdb=# select lp,lp_off,lp_flags,t_xmin,t_xmax,t_field3 as t_cid,t_ctid,t_infomask2,t_infomask from heap_page_items(get_raw_page('t_mvcc1',0));
lp | lp_off | lp_flags | t_xmin | t_xmax | t_cid | t_ctid | t_infomask2 | t_infomask
----+--------+----------+--------+--------+-------+--------+-------------+------------
1 | 8152 | 1 | 2300 | 2302 | 0 | (0,3) | 16386 | 258
2 | 8112 | 1 | 2301 | 2302 | 1 | (0,4) | 16386 | 258
3 | 8072 | 1 | 2302 | 0 | 0 | (0,3) | 32770 | 10242
4 | 8032 | 1 | 2302 | 0 | 1 | (0,4) | 32770 | 10242
(4 rows)可以看到原數據仍存在,但t_xmax值為2302,表示這兩行已被更新,同時t_ctid指向新的heap tuple.
1/2號tuple的t_infomask2是16386即0x4002 -> HEAP_HOT_UPDATED
t_infomask是258,即0x0102 -> HEAP_XMIN_COMMITTED | HEAP_HASVARWIDTH
3/4號tuple的t_infomask2是32770,即0x8002 -> HEAP_ONLY_TUPLE
t_infomask是10242,即0x2802 -> HEAP_UPDATED | HEAP_XMAX_INVALID | HEAP_HASVARWIDTH
更新數據(回滾事務)
testdb=# begin; BEGIN testdb=# testdb=# update t_mvcc1 set c2='C2_1' where c1 = 1; UPDATE 1 testdb=# update t_mvcc1 set c2='C2_2' where c1 = 2; UPDATE 1 testdb=# testdb=# rollback; ROLLBACK testdb=# select cmin,cmax,xmin,xmax,ctid,c1,c2 from t_mvcc1; cmin | cmax | xmin | xmax | ctid | c1 | c2 ------+------+------+------+-------+----+------ 0 | 0 | 2302 | 2303 | (0,3) | 1 | C2#1 1 | 1 | 2302 | 2303 | (0,4) | 2 | C2#2 (2 rows)
通過pageinspect插件查看page中的內容
testdb=# select lp,lp_off,lp_flags,t_xmin,t_xmax,t_field3 as t_cid,t_ctid,t_infomask2,t_infomask from heap_page_items(get_raw_page('t_mvcc1',0));
lp | lp_off | lp_flags | t_xmin | t_xmax | t_cid | t_ctid | t_infomask2 | t_infomask
----+--------+----------+--------+--------+-------+--------+-------------+------------
1 | 8152 | 1 | 2300 | 2302 | 0 | (0,3) | 16386 | 1282
2 | 8112 | 1 | 2301 | 2302 | 1 | (0,4) | 16386 | 1282
3 | 8072 | 1 | 2302 | 2303 | 0 | (0,5) | 49154 | 8450
4 | 8032 | 1 | 2302 | 2303 | 1 | (0,6) | 49154 | 8450
5 | 7992 | 1 | 2303 | 0 | 0 | (0,5) | 32770 | 10242
6 | 7952 | 1 | 2303 | 0 | 1 | (0,6) | 32770 | 10242
(6 rows)3/4號(lp=3/4)tuple被更新,t_xmax設置為更新事務的ID,但事務rollback(PG通過clog記錄事務狀態,clog后續再行討論).
t_infomask2=49154,即0xC002
t_infomask=8450,即0x2102 -> HEAP_UPDATED | HEAP_XMIN_COMMITTED | HEAP_HASVARWIDTH
5/6號tuple是新生成的更新記錄,但事務rollback.
t_infomask2=32770,即0x8002 -> HEAP_ONLY_TUPLE
t_infomask=10242,即0x2802 -> HEAP_UPDATED | HEAP_XMAX_INVALID | HEAP_HASVARWIDTH
刪除數據(提交事務)
testdb=# begin; BEGIN testdb=# testdb=# delete from t_mvcc1 where c1 = 1; DELETE 1 testdb=# testdb=# commit; COMMIT testdb=# testdb=# select cmin,cmax,xmin,xmax,ctid,c1,c2 from t_mvcc1; cmin | cmax | xmin | xmax | ctid | c1 | c2 ------+------+------+------+-------+----+------ 1 | 1 | 2302 | 2303 | (0,4) | 2 | C2#2 (1 row)
通過pageinspect插件查看page中的內容
testdb=# select lp,lp_off,lp_flags,t_xmin,t_xmax,t_field3 as t_cid,t_ctid,t_infomask2,t_infomask from heap_page_items(get_raw_page('t_mvcc1',0));
lp | lp_off | lp_flags | t_xmin | t_xmax | t_cid | t_ctid | t_infomask2 | t_infomask
----+--------+----------+--------+--------+-------+--------+-------------+------------
1 | 8152 | 1 | 2300 | 2302 | 0 | (0,3) | 16386 | 1282
2 | 8112 | 1 | 2301 | 2302 | 1 | (0,4) | 16386 | 1282
3 | 8072 | 1 | 2302 | 2304 | 0 | (0,3) | 40962 | 9474
4 | 8032 | 1 | 2302 | 2303 | 1 | (0,6) | 49154 | 10498
5 | 7992 | 1 | 2303 | 0 | 0 | (0,5) | 32770 | 10754
6 | 7952 | 1 | 2303 | 0 | 1 | (0,6) | 32770 | 10754
(6 rows)3號(lp=3) tuple被刪除,t_xmax修改為2304,t_ctid修改為(0,3).
t_infomask2=40962,即0xA002
t_infomask=9474,即0x2502 -> HEAP_UPDATED | HEAP_XMAX_COMMITTED | HEAP_XMIN_COMMITTED | HEAP_HASVARWIDTH
刪除數據(回滾事務)
testdb=# begin; BEGIN testdb=# testdb=# delete from t_mvcc1 where c1 = 2; DELETE 1 testdb=# testdb=# rollback; ROLLBACK testdb=# testdb=# select cmin,cmax,xmin,xmax,ctid,c1,c2 from t_mvcc1; cmin | cmax | xmin | xmax | ctid | c1 | c2 ------+------+------+------+-------+----+------ 0 | 0 | 2302 | 2305 | (0,4) | 2 | C2#2 (1 row)
xmax修改為事務號2305(原為2303).
通過pageinspect插件查看page中的內容
testdb=# select lp,lp_off,lp_flags,t_xmin,t_xmax,t_field3 as t_cid,t_ctid,t_infomask2,t_infomask from heap_page_items(get_raw_page('t_mvcc1',0));
lp | lp_off | lp_flags | t_xmin | t_xmax | t_cid | t_ctid | t_infomask2 | t_infomask
----+--------+----------+--------+--------+-------+--------+-------------+------------
1 | 8152 | 1 | 2300 | 2302 | 0 | (0,3) | 16386 | 1282
2 | 8112 | 1 | 2301 | 2302 | 1 | (0,4) | 16386 | 1282
3 | 8072 | 1 | 2302 | 2304 | 0 | (0,3) | 40962 | 9474
4 | 8032 | 1 | 2302 | 2305 | 0 | (0,4) | 40962 | 10498
5 | 7992 | 1 | 2303 | 0 | 0 | (0,5) | 32770 | 10754
6 | 7952 | 1 | 2303 | 0 | 1 | (0,6) | 32770 | 10754
(6 rows)刪除4號(lp=4) tuple,但事務回滾,t_max修改為2305.
t_infomask2=40962,即0xA002
t_infomask=10498,即0x2902 -> HEAP_UPDATED | HEAP_XMAX_INVALID | HEAP_XMIN_COMMITTED | HEAP_HASVARWIDTH
到此，關于“PostgreSQL中的Multi Version Heap Tuple分析”的學習就結束了，希望能夠解決大家的疑惑。理論與實踐的搭配能更好的幫助大家學習，快去試試吧！若想繼續學習更多相關知識，請繼續關注億速云網站，小編會繼續努力為大家帶來更多實用的文章！
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