Sql-Server-2016

游標 sys.databases 會跳過數據庫

  • May 24, 2021

我知道這個問題已經被問過好幾次了,我知道解決方案,但我試圖了解問題的根本原因:

我有以下程式碼來執行數據庫備份。

DECLARE @Filename   VARCHAR(256)
DECLARE @FileDate   VARCHAR(15) 
DECLARE @Path       VARCHAR(50) 
DECLARE @Name       VARCHAR(50) 

-- specify database backup directory
SET @Path = '\MyPath'

-- specify filename date
SELECT @FileDate = CONVERT(VARCHAR(20), GETDATE(), 112) + '_' + REPLACE(CONVERT(VARCHAR(20), GETDATE(), 108),':','')

DECLARE db_cursor CURSOR FOR
   SELECT [name]
       FROM master.sys.databases
       WHERE [name] NOT IN ('master', 'msdb', 'model', 'tempdb')
           AND [state_desc] = 'ONLINE'

OPEN db_cursor
FETCH NEXT FROM db_cursor INTO @Name

WHILE @@FETCH_STATUS = 0
BEGIN

   SET @Filename = @Path + @Name + '_Full_Backup_' + @FileDate + '.bak'
   BACKUP DATABASE @Name
   TO DISK = @Filename
   WITH CHECKSUM,
       COMPRESSION

   FETCH NEXT FROM db_cursor INTO @Name
END

CLOSE db_cursor
DEALLOCATE db_cursor

有時,只備份了一些數據庫,這表明游標沒有遍歷所有返回的行,或者查詢本身沒有返回它應該返回的所有數據庫的名稱。

我試圖理解為什麼會發生這種情況。我知道解決方法是使用STATIC游標,這表明問題出在基礎查詢中的結果上

SELECT [name]
   FROM master.sys.databases
   WHERE [name] NOT IN ('master', 'msdb', 'model', 'tempdb')
       AND [state_desc] = 'ONLINE'

正在改變,但我看不到會發生什麼變化(沒有數據庫名稱會改變,並且錯誤日誌並不表明數據庫狀態已經改變)

sys.databases是一個複雜的視圖。在 SQL Server 2016 上是:

CREATE VIEW sys.databases AS
   SELECT d.name, 
       d.id AS database_id,
       r.indepid AS source_database_id,
       d.sid AS owner_sid,
       d.crdate AS create_date,
       d.cmptlevel AS compatibility_level,
       -- coll.value = null means that a collation was not specified for the DB and the server default is used instead
       convert(sysname, case when serverproperty('EngineEdition') = 5 AND d.id = 1 then serverproperty('collation')
                                else CollationPropertyFromID(convert(int, isnull(coll.value, p.cid)), 'name') end) AS collation_name,
       iif ((serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x10000000) = 1), cast (3 as tinyint), p.user_access) AS user_access,
       iif ((serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x10000000) = 1), 'NO_ACCESS', ua.name) AS user_access_desc,
       sysconv(bit, d.status & 0x400) AS is_read_only,         -- DBR_RDONLY
       sysconv(bit, d.status & 1) AS is_auto_close_on,         -- DBR_CLOSE_ON_EXIT
       sysconv(bit, d.status & 0x400000) AS is_auto_shrink_on,     -- DBR_AUTOSHRINK
       case when (serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x00000020) = 1) then cast (1 as tinyint) -- RESTORING
            when (serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x00000080) = 1) then cast (7 as tinyint) -- COPYING
            when (serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x00000100) = 1) then cast (4 as tinyint) -- SUSPECT
            when (serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x08000000) = 1) then cast (8 as tinyint) -- QUORUM_RECOVERY_PENDING
            when (serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x04000000) = 1) then cast (9 as tinyint) -- CREATING
            else p.state 
            end AS state, -- 7 is COPYING and 4 is SUSPECT state for database copy (UNDO: Need to have a clean way to set states in dbtable for a user db)
       case when (serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x00000020) = 1) then 'RESTORING' 
            when (serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x00000080) = 1) then 'COPYING' 
            when (serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x00000100) = 1) then 'SUSPECT'
            when (serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x08000000) = 1) then CONVERT(nvarchar(60), N'QUORUM_RECOVERY_PENDING')
            when (serverproperty('EngineEdition') = 5) AND (sysconv(bit, d.status & 0x04000000) = 1) then 'CREATING'
            else st.name 
            end AS state_desc,
       sysconv(bit, d.status & 0x200000) AS is_in_standby,     -- DBR_STANDBY
       case when serverproperty('EngineEdition') = 5 then convert(bit, 0) else p.is_cleanly_shutdown end AS is_cleanly_shutdown,
       sysconv(bit, d.status & 0x80000000) AS is_supplemental_logging_enabled, -- DBR_SUPPLEMENT_LOG
       case when (serverproperty('EngineEdition') = 5) then sysconv(tinyint, sysconv(bit,(d.status & 0x00100000)))
            else p.snapshot_isolation_state end AS snapshot_isolation_state,
       case when (serverproperty('EngineEdition') = 5) and (sysconv(bit, d.status & 0x00100000) = 1) then 'ON'
            when (serverproperty('EngineEdition') = 5) and (sysconv(bit, d.status & 0x00100000) = 0) then 'OFF'
            else si.name end AS snapshot_isolation_state_desc,     
       sysconv(bit, d.status & 0x800000) AS is_read_committed_snapshot_on,     -- DBR_READCOMMITTED_SNAPSHOT
       case when (serverproperty('EngineEdition') = 5) 
            then case 
                   when sysconv(bit,(d.status & 0x00000008)) = 1
                       then cast(3 as tinyint)
                   when sysconv(bit,(d.status & 0x00000004)) = 1
                       then cast(2 as tinyint)
                   else
                       cast(1 as tinyint)
                  end  
             else p.recovery_model 
       end AS recovery_model,
       case when (serverproperty('EngineEdition') = 5) 
            then case 
                   when sysconv(bit,(d.status & 0x00000008)) = 1
                       then CONVERT(nvarchar(60), N'SIMPLE')
                   when sysconv(bit,(d.status & 0x00000004)) = 1
                       then CONVERT(nvarchar(60), N'BULK_LOGGED')
                   else
                       CONVERT(nvarchar(60), N'FULL')
                 end   
             else ro.name 
       end AS recovery_model_desc,              
       p.page_verify_option, pv.name AS page_verify_option_desc,
       sysconv(bit, d.status2 & 0x1000000) AS is_auto_create_stats_on,         -- DBR_AUTOCRTSTATS
       sysconv(bit, d.status2 & 0x00400000) AS is_auto_create_stats_incremental_on,    -- DBR_AUTOCRTSTATSINC
       sysconv(bit, d.status2 & 0x40000000) AS is_auto_update_stats_on,        -- DBR_AUTOUPDSTATS
       sysconv(bit, d.status2 & 0x80000000) AS is_auto_update_stats_async_on,  -- DBR_AUTOUPDSTATSASYNC
       sysconv(bit, d.status2 & 0x4000) AS is_ansi_null_default_on,            -- DBR_ANSINULLDFLT
       sysconv(bit, d.status2 & 0x4000000) AS is_ansi_nulls_on,                -- DBR_ANSINULLS
       sysconv(bit, d.status2 & 0x2000) AS is_ansi_padding_on,                 -- DBR_ANSIPADDING
       sysconv(bit, d.status2 & 0x10000000) AS is_ansi_warnings_on,            -- DBR_ANSIWARNINGS
       sysconv(bit, d.status2 & 0x1000) AS is_arithabort_on,                   -- DBR_ARITHABORT
       sysconv(bit, d.status2 & 0x10000) AS is_concat_null_yields_null_on,     -- DBR_CATNULL
       sysconv(bit, d.status2 & 0x800) AS is_numeric_roundabort_on,            -- DBR_NUMEABORT
       sysconv(bit, d.status2 & 0x800000) AS is_quoted_identifier_on,          -- DBR_QUOTEDIDENT
       sysconv(bit, d.status2 & 0x20000) AS is_recursive_triggers_on,          -- DBR_RECURTRIG
       sysconv(bit, d.status2 & 0x2000000) AS is_cursor_close_on_commit_on,    -- DBR_CURSCLOSEONCOM
       sysconv(bit, d.status2 & 0x100000) AS is_local_cursor_default,          -- DBR_DEFLOCALCURS
       sysconv(bit, d.status2 & 0x20000000) AS is_fulltext_enabled,            -- DBR_FTENABLED
       sysconv(bit, d.status2 & 0x200) AS is_trustworthy_on,               -- DBR_TRUSTWORTHY
       sysconv(bit, d.status2 & 0x400) AS is_db_chaining_on,               -- DBR_DBCHAINING
       sysconv(bit, d.status2 & 0x08000000) AS is_parameterization_forced, -- DBR_UNIVERSALAUTOPARAM
       sysconv(bit, d.status2 & 64) AS is_master_key_encrypted_by_server,  -- DBR_MASTKEY
       sysconv(bit, d.status2 & 0x00000010) AS is_query_store_on,          -- DBR_QDSENABLED
       sysconv(bit, d.category & 1) AS is_published,
       sysconv(bit, d.category & 2) AS is_subscribed,
       sysconv(bit, d.category & 4) AS is_merge_published,
       sysconv(bit, d.category & 16) AS is_distributor,
       sysconv(bit, d.category & 32) AS is_sync_with_backup,
       d.svcbrkrguid AS service_broker_guid,
       sysconv(bit, case when d.scope = 0 then 1 else 0 end) AS is_broker_enabled,
       p.log_reuse_wait, lr.name AS log_reuse_wait_desc,
       sysconv(bit, d.status2 & 4) AS is_date_correlation_on,      -- DBR_DATECORRELATIONOPT
       sysconv(bit, d.category & 64) AS is_cdc_enabled,
       case 
           when (d.id = db_id('tempdb')) then sysconv(bit, p.is_db_encrypted)
           else sysconv(bit, d.status2 & 0x100)                    -- DBR_ENCRYPTION
           end AS is_encrypted,
       convert(bit, d.status2 & 0x8) AS is_honor_broker_priority_on,               -- DBR_HONORBRKPRI
       sgr.guid AS replica_id,
       sgr2.guid AS group_database_id,
       ssr.indepid AS resource_pool_id,
       default_language_lcid = case when ((d.status2 & 0x80000)=0x80000 AND p.containment = 1) then convert(smallint, p.default_language) else null end,
       default_language_name = case when ((d.status2 & 0x80000)=0x80000 AND p.containment = 1) then convert(sysname, sld.name) else null end,
       default_fulltext_language_lcid = case when ((d.status2 & 0x80000)=0x80000 AND p.containment = 1) then convert(int, p.default_fulltext_language) else null end,
       default_fulltext_language_name = case when ((d.status2 & 0x80000)=0x80000 AND p.containment = 1) then convert(sysname, slft.name) else null end,
       is_nested_triggers_on = case when ((d.status2 & 0x80000)=0x80000 AND p.containment = 1) then convert(bit, p.allow_nested_triggers) else null end,
       is_transform_noise_words_on = case when ((d.status2 & 0x80000)=0x80000 AND p.containment = 1) then convert(bit, p.transform_noise_words) else null end,
       two_digit_year_cutoff = case when ((d.status2 & 0x80000)=0x80000 AND p.containment = 1) then convert(smallint, p.two_digit_year_cutoff) else null end,
       containment = sysconv(tinyint, (d.status2 & 0x80000)/0x80000), -- DBR_IS_CDB
       containment_desc = convert(nvarchar(60), cdb.name),
       p.recovery_seconds AS target_recovery_time_in_seconds,
       p.delayed_durability,
       case when (p.delayed_durability = 0) then CAST('DISABLED' AS nvarchar(60)) -- LCOPT_DISABLED
            when (p.delayed_durability = 1) then CAST('ALLOWED' AS nvarchar(60)) -- LCOPT_ALLOWED
            when (p.delayed_durability = 2) then CAST('FORCED' AS nvarchar(60)) -- LCOPT_FORCED
            else NULL
            end AS delayed_durability_desc,
       convert(bit, d.status2 & 0x80) AS 
       is_memory_optimized_elevate_to_snapshot_on,             -- DBR_HKELEVATETOSNAPSHOT
       sysconv(bit, d.category & 0x100) AS is_federation_member,
       convert(bit, isnull(rda.value, 0)) AS is_remote_data_archive_enabled,
       convert(bit, p.is_mixed_page_allocation_on) AS is_mixed_page_allocation_on
   FROM sys.sysdbreg d OUTER APPLY OpenRowset(TABLE DBPROP, (case when serverproperty('EngineEdition') = 5 then DB_ID() else d.id end)) p
   LEFT JOIN sys.syssingleobjrefs r ON r.depid = d.id AND r.class = 96 AND r.depsubid = 0  -- SRC_VIEWPOINTDB
   LEFT JOIN sys.syspalvalues st ON st.class = 'DBST' AND st.value = p.state
   LEFT JOIN sys.syspalvalues ua ON ua.class = 'DBUA' AND ua.value = p.user_access
   LEFT JOIN sys.syspalvalues si ON si.class = 'DBSI' AND si.value = p.snapshot_isolation_state
   LEFT JOIN sys.syspalvalues ro ON ro.class = 'DBRO' AND ro.value = p.recovery_model
   LEFT JOIN sys.syspalvalues pv ON pv.class = 'DBPV' AND pv.value = p.page_verify_option
   LEFT JOIN sys.syspalvalues lr ON lr.class = 'LRWT' AND lr.value = p.log_reuse_wait
   LEFT JOIN sys.syssingleobjrefs agdb ON agdb.depid = d.id AND agdb.class = 104 AND agdb.depsubid = 0 -- SRC_AVAILABILITYGROUP 
   LEFT JOIN master.sys.syssingleobjrefs ssr ON ssr.class = 108 AND ssr.depid = d.id -- SRC_RG_DB_TO_POOL
   LEFT JOIN master.sys.sysclsobjs  ag ON ag.id = agdb.indepid AND ag.class = 67 -- SOC_AVAILABILITY_GROUP
   LEFT JOIN master.sys.sysguidrefs sgr ON sgr.class = 8 AND sgr.id = ag.id AND sgr.subid = 1 -- GRC_AGGUID / AGGUID_REPLICA_ID
   LEFT JOIN master.sys.sysguidrefs sgr2 ON sgr2.class = 9 AND sgr2.id = ag.id AND sgr2.subid = d.id -- GRC_AGDBGUID
   LEFT JOIN sys.syspalvalues cdb ON cdb.class = 'DCDB' AND cdb.value = CASE WHEN (d.status2 & 0x80000)=0x80000 THEN 1 ELSE 0 END
   LEFT JOIN sys.syslanguages sld ON sld.lcid = p.default_language
   LEFT JOIN sys.fulltext_languages slft ON slft.lcid = p.default_fulltext_language
   LEFT JOIN sys.sysobjvalues coll ON coll.valclass = 102 AND coll.subobjid = 0 AND coll.objid = d.id  -- SVC_DATACOLLATION
   LEFT JOIN sys.sysobjvalues rda ON rda.valclass = 116 AND rda.objid = d.id AND rda.valnum = 0 -- SVC_STRETCH & STRETCH_DB_IS_STRETCHED
   WHERE d.id < 0x7fff
       AND has_access('DB', (case when serverproperty('EngineEdition') = 5 then DB_ID() else d.id end)) = 1

通過不指定特定的游標選項,您隱式地請求local, dynamic, updatable, optimistic, forward-only此視圖上的游標。

SQL Server 無法生成動態游標計劃,因此它將游標轉換為本地、鍵集、可更新、樂觀、只進游標。

鍵集意味著在打開游標時,在底層系統表中定位行所需的最少鍵儲存在tempdb中。如果底層系統表中的這些鍵值中的任何一個發生更改,則不會返回一行並將@@FETCH_STATUS返回 -2。您的循環將因失敗而過早退出@@FETCH_STATUS = 0,並且不會處理游標中的其他數據庫。可能導緻密鑰更改的一個範例是更改ALLOW_SNAPSHOT_ISOLATION數據庫的狀態。

當可能發生並發鍵更改時,鍵集游標是不明智的。我們不控制基礎表,因此使用鍵集游標sys.databases是自找麻煩。檢查失敗也很重要@@FETCH_STATUS,而不是在到達終點之前假設成功。

樂觀游標使用校驗和來檢測自游標打開以來對行的更改,因此不會失去更新。這在這裡並不直接重要,因為您不是通過游標更新行,但 SQL Server 不知道這一點。它必須制定一個收集、儲存和比較校驗和的計劃。

旁注:在 open 和 fetch 執行計劃中完成的大部分工作都是多餘的,因為您只對數據庫名稱和state_desc列感興趣。大多數冗餘系統表訪問無法刪除,因為優化器沒有足夠的鍵關係資訊來允許刪除外連接。您可以通過添加來解決此問題DISTINCT name(因此外部連接不能重複行),但這也會強制游標降級為靜態(快照)。不過,您可能有興趣查看添加DISTINCT.

這裡的教訓是明確指定您想要的游標類型,而不是依賴預設值。游標的這種使用要求靜態(快照)類型:

DECLARE db_cursor CURSOR 
   LOCAL FORWARD_ONLY STATIC READ_ONLY
   FOR ...

引用自:https://dba.stackexchange.com/questions/261540