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Home

Technical Articles

Database

Oracle

How to find the size of a table in Oracle?

To find the size of an Oracle table, you need to query the size of its related segments. The specific steps are as follows: 1. Use the USER_SEGMENTS or DBA_SEGMENTS view to get the size of the table and its related objects, execute the SELECT statement and replace the table name in capital form to get the size in MB; 2. By grouping the query by segment_type, you can view the size of the table data, index and LOB segments respectively; 3. To obtain the total space occupation of the table and all related objects (including index and LOB), you need to jointly query the segment names in user_segments, user_indexes and user_lobs; 4. If you only need the table data size, you can add it in the query

Jul 26, 2025 am 02:45 AM

Why is gathering accurate statistics crucial for Oracle Optimizer performance?

OracleOptimizerrequiresaccuratestatisticstogenerateefficientexecutionplansbecauseoutdatedorincorrectstatscanleadtopoorqueryperformance.1.Theoptimizerusesstatisticstoestimatetablesize,indexselectivity,androwcounts,whichdetermineoperationslikeindexusag

Jul 25, 2025 am 01:20 AM

Can you describe the basic structure of a PL/SQL block (DECLARE, BEGIN, EXCEPTION, END)?

The structure of PL/SQL block mainly includes four parts: DECLARE, BEGIN, EXCEPTION and END. 1.DECLARE is an optional part, used to declare variables, constants, cursors and user-defined types, and all declarations must be before the BEGIN keyword; 2.BEGIN is a required part, containing execution logic, such as SQL statements, control structures and function calls, which are where the actual work is completed; 3.EXCEPTION is an optional error handling part, used to handle runtime errors, and supports specific exceptions (such as NO_DATA_FOUND) and general exceptions (WHENOTHERSTHEN); 4.END marks the end of the block, and each PL/SQL block must be

Jul 25, 2025 am 12:57 AM

structure pl/sql

What is the Oracle Optimizer, and how does it determine the execution plan for a SQL statement?

TheOracleOptimizerdeterminesthemostefficientwaytoexecuteSQLbyanalyzingexecutionplansbasedonstatisticsandcostestimation.1.Itdecideshowtoaccessdata,includingindexusage,tablejoinorder,andjoinmethods.2.Itestimatescostusingtableandsystemstatistics,andpred

Jul 25, 2025 am 12:47 AM

SQL執(zhí)行計(jì)劃

What are global temporary tables in Oracle?

GlobaltemporarytablesinOracleprovidesession-privateortransaction-privatetemporarystoragewithsharedstructure,whereeachsessionseesonlyitsowndata:1)Thetabledefinitionisglobalandaccessibletoallsessions;2)Datainsertionandretrievalareisolatedpersession;3)D

Jul 25, 2025 am 12:25 AM

How to query hierarchical data in Oracle using CONNECT BY?

When querying Oracle hierarchical data using CONNECTBY, you need to use STARTWITH to specify the root node, CONNECTBYPRIOR defines the parent-child relationship, and LEVEL represents the hierarchy depth; 2. In the example, the employee hierarchy structure is constructed through manager_id and emp_id, and use LPAD to realize indentation display; 3. Use LEVEL, CONNECT_BY_ROOT and SYS_CONNECT_BY_PATH to obtain the node depth, root node value and the path from the root to the current node; 4. The filtering conditions act on the result in WHERE, and in STARTWITH or CONNECTBY will affect the traversal process; 5. When there is a loop, NO should be used.

Jul 24, 2025 am 04:06 AM

oracle 層次數(shù)據(jù)

How does Oracle Data Guard provide disaster recovery and high availability solutions?

OracleDataGuardensureshighavailabilityanddisasterrecoverybyreplicatingdatainreal-timetostandbydatabases,enablingautomaticfailover,offeringmultipleprotectionmodes,andsupportingroletransitions.1.Itcontinuouslyreplicatesdatafromtheprimarytooneormorestan

Jul 24, 2025 am 03:58 AM

disaster recovery

How to tune a slow query in Oracle?

IdentifytheslowqueryusingV$SQL,AWR,orTKPROFandanalyzeitsexecutionplanviaDBMS_XPLANtospotfulltablescans,high-costoperations,orcardinalitymisestimates;2.EvaluateandcreatemissingorefficientindexesonWHERE,JOIN,andORDERBYcolumns,avoidfunction-baseddisable

Jul 24, 2025 am 03:14 AM

slow query Oracle調(diào)優(yōu)

How to kill a session in Oracle?

To terminate an Oracle session, first query the SID and SERIAL# through the V$SESSION view to locate the target session, and then execute the ALTERSYSTEMKILLSESSION'sid, serial#' command to terminate; if the session does not respond, the IMMEDIATE keyword can be added, and it is only forced to terminate by SPID at the operating system level in extreme cases. It should be noted that the transaction will be rolled back after the session is terminated, so be sure to confirm that the session information is accurate before performing the operation.

Jul 24, 2025 am 01:31 AM

What is the purpose of the Oracle Resource Manager, and how can it manage database resources among users?

OracleResourceManager achieves resource optimization by dividing user groups, limiting concurrency and resource consumption, dynamic switching strategies, and monitoring and adjustment. 1. It groups users (such as REPORTING_GROUP, DEVELOPER_GROUP), allocates CPU resources according to priority, ensuring that critical tasks obtain higher quotas; 2. Controls the number of concurrent sessions, execution time and I/O upper limit to prevent system overload; 3. Automatically switch resource plans according to time periods, such as focusing on OLTP during the day and batch processing at night; 4. Provides views such as V$RSRC_CONSUMER_GROUP to monitor usage, and it is recommended to regularly check and adjust strategies to improve performance.

Jul 23, 2025 am 04:19 AM

數(shù)據(jù)庫資源管理

How do Oracle sequences differ from identity columns (introduced in later versions)?

Oraclesequences and identitycolumns can generate self-value-added, but the mechanism is different from the applicable scenarios. 1. Oracle sequences are independent objects that can be used across tables, providing higher control flexibility, such as cache, looping, etc.; 2. The Identity column embeds the self-increment logic in the table column, simplifying the settings, suitable for simple scenarios and closer to MySQL/PostgreSQL usage; 3. The key differences are the scope of action (sequences are available globally, identity columns are limited to single tables), control ability (sequence functions are stronger) and ease of use (identity columns are more intuitive); 4. It is recommended to use identity columns in simple scenarios, and when complex systems or shared counters are required, the sequence is preferred.

Jul 23, 2025 am 04:17 AM

How does Oracle manage undo data, and what is the role of the undo tablespace?

Oracle uses undotablespace to store data images before transaction modification to support transaction consistency, rollback operations, and database-level read consistency. Its core functions include: 1. Rolling back uncommitted transactions; 2. Provide a consistent data read view; 3. Support recovery after instance or media failure; 4. Ensure ACID transaction properties. Oracle automatically manages the revocation segment and configures retention time through UNDO_RETENTION, while recycling the revocation data of the committed transaction when there is insufficient space. Common problems include ORA-01555 errors, high undo generation rates, and too small undo tablespaces. Therefore, the undo tablespace size should be set reasonably based on the workload. establish

Jul 23, 2025 am 04:04 AM

oracle

How do you perform a graceful shutdown versus an abort of an Oracle instance?

Oracle instances should prioritize elegant shutdown using SHUTDOWNIMMEDIATE, and use SHUTDOWNABORT only when the database is unresponsive. 1.SHUTDOWNIMMEDIATE will block new connections, roll back active transactions, disconnect all sessions, and close database files normally to ensure data consistency; 2.SHUTDOWNABORT will directly terminate the instance, and will not roll back transactions or write to disk, resulting in instance recovery required for next startup; 3. Key differences include transaction rollback behavior, data integrity guarantee and startup impact; 4. Best practice is always the first choice to close immediately unless the database is completely stuck or is in a non-critical test environment.

Jul 23, 2025 am 02:41 AM

How can Oracle Advanced Queuing (AQ) be used for message-based communication?

OracleAdvancedQueuing(AQ)enablesasynchronousmessage-basedcommunicationwithinOracleDatabasethroughbuilt-inqueues.1.SetupqueuetablesanddefinemessagetypesusingDBMS_AQADMprocedurestocreatestructuredqueues.2.EnqueuemessageswithDBMS_AQ.ENQUEUE,optionallysp

Jul 22, 2025 am 03:54 AM

message queue