手動執行VACUUM，無法清理表中的dead tuple
目錄
環境
系統平臺：N/A
版本：4.1.1
症狀
應用方反映業務量不大的情況下伺服器負載較高，執行SQL的速度明顯感覺緩慢。檢查資料庫TOP SQL，發現與表test有關，檢視該表的統計值，live_tuple只有91966條，dead tuple為702346條。
| highgo=# select n_live_tup,n_dead_tup from pg_stat_all_tables where relname = 'test'; n_live_tup | n_dead_tup ------------+------------ 91966 | 702346 (1 row) |
手動對該表執行VACUUM同樣無法回收dead tuple，報錯如下：
| highgo=# vacuum(verbose,analyze) test; INFO: 00000: vacuuming "public.test" INFO: 00000: index "index_test_content_id" now contains 146076 row versions in 698 pages DETAIL: 0 index row versions were removed. 175 index pages have been deleted, 126 are currently reusable. CPU 0.00s/0.00u sec elapsed 0.00 sec. INFO: 00000: index "test_pkey" now contains 146076 row versions in 568 pages DETAIL: 0 index row versions were removed. 77 index pages have been deleted, 68 are currently reusable. CPU 0.00s/0.00u sec elapsed 0.00 sec. INFO: 00000: "test": found 0 removable, 789485 nonremovable row versions in 10178 out of 10276 pages DETAIL: 702346 dead row versions cannot be removed yet. There were 59707 unused item pointers. Skipped 0 pages due to buffer pins. 0 pages are entirely empty. CPU 0.04s/0.15u sec elapsed 0.20 sec. INFO: 00000: vacuuming "pg_toast.pg_toast_16737" INFO: 00000: index "pg_toast_16737_index" now contains 0 row versions in 1 pages DETAIL: 0 index row versions were removed. 0 index pages have been deleted, 0 are currently reusable. CPU 0.00s/0.00u sec elapsed 0.13 sec. INFO: 00000: "pg_toast_16737": found 0 removable, 0 nonremovable row versions in 0 out of 0 pages DETAIL: 0 dead row versions cannot be removed yet. There were 0 unused item pointers. Skipped 0 pages due to buffer pins. 0 pages are entirely empty. CPU 0.00s/0.00u sec elapsed 0.59 sec. INFO: 00000: analyzing "public.test" INFO: 00000: "test": scanned 10276 of 10276 pages, containing 90959 live rows and 702346 dead rows; 30000 rows in sample, 90959 estimated total row |
問題原因
當我們開啟一個事務後，為了實現多版本的控制，資料庫會阻止未結束的長事務之後產生的所有dead tuple,即使與該事務無關的其他表也是如此。
- Live transactions performing a write operation in any table will prevent vacuuming dead rows
generated by commited transactions that started after first live transaction in any other table.
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https://support.highgo.com/#/index/docContent/35b88a720452d610