PostgreSQL 手册
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前言何为PostgreSQL？PostgreSQL简史格式约定更多信息臭虫汇报指导I. 教程章1. 从头开始1.1. 安装1.2. 体系基本概念1.3. 创建一个数据库1.4. 访问数据库章2. SQL语言2.1. 介绍2.2. 概念2.3. 创建新表2.4. 向表中添加行2.5. 查询一个表2.6. 表间链接2.7. 聚集函数2.8. 更新2.9. 删除章3. 高级特性3.1. 介绍3.2. 视图3.3. 外键3.4. 事务3.5. 窗口函数3.6. 继承3.7. 结论II. SQL语言章4. SQL语法4.1. 词法结构4.2. 值表达式4.3. 调用函数章5. 数据定义5.1. 表的基本概念5.2. 缺省值5.3. 约束5.4. 系统字段5.5. 修改表5.6. 权限5.7. 模式5.8. 继承5.9. 分区5.10. 其它数据库对象5.11. 依赖性跟踪章 6. 数据操作6.1. 插入数据6.2. 更新数据6.3. 删除数据章7. 查询7.1. 概述7.2. 表表达式7.3. 选择列表7.4. 组合查询7.5. 行排序7.6. LIMIT和OFFSET7.7. VALUES列表7.8. WITH的查询（公用表表达式）章8. 数据类型8.1. 数值类型8.2. 货币类型8.3. 字符类型8.4. 二进制数据类型8.5. 日期/时间类型8.6. 布尔类型8.7. 枚举类型8.8. 几何类型8.9. 网络地址类型8.10. 位串类型8.11. 文本搜索类型8.12. UUID类型8.13. XML类型8.14. 数组8.15. 复合类型8.16. 对象标识符类型8.17. 伪类型章 9. 函数和操作符9.1. 逻辑操作符9.2. 比较操作符9.3. 数学函数和操作符9.4. 字符串函数和操作符9.5. 二进制字符串函数和操作符9.6. 位串函数和操作符9.7. 模式匹配9.8. 数据类型格式化函数9.9. 时间/日期函数和操作符9.10. 支持枚举函数9.11. 几何函数和操作符9.12. 网络地址函数和操作符9.13. 文本检索函数和操作符9.14. XML函数9.15. 序列操作函数9.16. 条件表达式9.17. 数组函数和操作符9.18. 聚合函数9.19. 窗口函数9.20. 子查询表达式9.21. 行和数组比较9.22. 返回集合的函数9.23. 系统信息函数9.24. 系统管理函数9.25. 触发器函数章10. 类型转换10.3. 函数10.2. 操作符10.1. 概述10.4. 值存储10.5. UNION章11. 索引11.1. 介绍11.2. 索引类型11.3. 多字段索引11.4. 索引和ORDER BY11.5. 组合多个索引11.6. 唯一索引11.7. 表达式上的索引11.8. 部分索引11.9. 操作类和操作簇11.10. 检查索引的使用章12. Full Text Search12.1. Introduction12.2. Tables and Indexes12.3. Controlling Text Search12.4. Additional Features12.5. Parsers12.6. Dictionaries12.7. Configuration Example12.8. Testing and Debugging Text Search12.9. GiST and GIN Index Types12.10. psql Support12.11. Limitations12.12. Migration from Pre-8.3 Text Search章13. 并发控制13.1. 介绍13.2. 事务隔离13.3. 明确锁定13.4. 应用层数据完整性检查13.5. 锁和索引章14. 性能提升技巧14.1. 使用EXPLAIN14.2. 规划器使用的统计信息14.3. 用明确的JOIN语句控制规划器14.4. 向数据库中添加记录14.5. 非持久性设置III. 服务器管理章15. 安装指导15.1. 简版15.2. 要求15.3. 获取源码15.4. 升级15.5. 安装过程15.6. 安装后的设置15.7. 支持的平台15.8. 特殊平台的要求章16. Installation from Source Code on Windows16.1. Building with Visual C++ or the Platform SDK16.2. Building libpq with Visual C++ or Borland C++章17. 服务器安装和操作17.1. PostgreSQL用户帐户17.2. 创建数据库集群17.3. 启动数据库服务器17.4. 管理内核资源17.5. 关闭服务17.6. 防止服务器欺骗17.7. 加密选项17.8. 用SSL进行安全的TCP/IP连接17.9. Secure TCP/IP Connections with SSH Tunnels章18. 服务器配置18.1. 设置参数18.2. 文件位置18.3. 连接和认证18.4. 资源消耗18.5. 预写式日志18.6. 查询规划18.7. 错误报告和日志18.8. 运行时统计18.9. 自动清理18.10. 客户端连接缺省18.12. 版本和平台兼容性18.11. 锁管理18.13. 预置选项18.14. 自定义的选项18.15. 开发人员选项18.16. 短选项章19. 用户认证19.1. pg_hba.conf 文件19.2. 用户名映射19.3. 认证方法19.4. 用户认证章20. 数据库角色和权限20.1. 数据库角色20.2. 角色属性20.3. 权限20.4. 角色成员20.5. 函数和触发器章21. 管理数据库21.1. 概述21.2. 创建一个数据库21.3. 临时库21.4. 数据库配置21.5. 删除数据库21.6. 表空间章22. 本土化22.1. 区域支持22.2. 字符集支持章23. 日常数据库维护工作23.1. Routine Vacuuming日常清理23.2. 经常重建索引23.3. 日志文件维护章24. 备份和恢复24.1. SQL转储24.2. 文件系统级别的备份24.3. 在线备份以及即时恢复(PITR)24.4. 版本间迁移章25. 高可用性与负载均衡，复制25.1. 不同解决方案的比较25.2. 日志传送备份服务器25.3. 失效切换25.4. 日志传送的替代方法25.5. 热备章26. 恢复配置26.1. 归档恢复设置26.2. 恢复目标设置26.3. 备服务器设置章27. 监控数据库的活动27.1. 标准Unix工具27.2. 统计收集器27.3. 查看锁27.4. 动态跟踪章28. 监控磁盘使用情况28.1. 判断磁盘的使用量28.2. 磁盘满导致的失效章29. 可靠性和预写式日志29.1. 可靠性29.2. 预写式日志(WAL)29.3. 异步提交29.4. WAL配置29.5. WAL内部章30. Regression Tests30.1. Running the Tests30.2. Test Evaluation30.3. Variant Comparison Files30.4. Test Coverage ExaminationIV. 客户端接口章31. libpq-C库31.1. 数据库联接函数31.2. 连接状态函数31.3. 命令执行函数31.4. 异步命令处理31.5. 取消正在处理的查询31.6. 捷径接口31.7. 异步通知31.8. 与COPY命令相关的函数31.9. Control Functions 控制函数31.10. 其他函数31.11. 注意信息处理31.12. 事件系统31.13. 环境变量31.14. 口令文件31.15. 连接服务的文件31.16. LDAP查找连接参数31.17. SSL支持31.18. 在多线程程序里的行为31.19. 制作libpq程序31.20. 例子程序章32. 大对象32.1. 介绍32.2. 实现特点32.3. 客户端接口32.4. 服务器端函数32.5. 例子程序章33. ECPG - Embedded SQL in C33.1. The Concept33.2. Connecting to the Database Server33.3. Closing a Connection33.4. Running SQL Commands33.5. Choosing a Connection33.6. Using Host Variables33.7. Dynamic SQL33.8. pgtypes library33.9. Using Descriptor Areas33.10. Informix compatibility mode33.11. Error Handling33.12. Preprocessor directives33.13. Processing Embedded SQL Programs33.14. Library Functions33.15. Internals章34. 信息模式34.1. 关于这个模式34.2. 数据类型34.3. information_schema_catalog_name34.4. administrable_role_authorizations34.5. applicable_roles34.6. attributes34.7. check_constraint_routine_usage34.8. check_constraints34.9. column_domain_usage34.10. column_privileges34.11. column_udt_usage34.12. 字段34.13. constraint_column_usage34.14. constraint_table_usage34.15. data_type_privileges34.16. domain_constraints34.18. domains34.17. domain_udt_usage34.19. element_types34.20. enabled_roles34.21. foreign_data_wrapper_options34.22. foreign_data_wrappers34.23. foreign_server_options34.24. foreign_servers34.25. key_column_usage34.26. parameters34.27. referential_constraints34.28. role_column_grants34.29. role_routine_grants34.30. role_table_grants34.31. role_usage_grants34.32. routine_privileges34.33. routines34.34. schemata34.35. sequences34.36. sql_features34.37. sql_implementation_info34.38. sql_languages34.39. sql_packages34.40. sql_parts34.41. sql_sizing34.42. sql_sizing_profiles34.43. table_constraints34.44. table_privileges34.45. tables34.46. triggered_update_columns34.47. 触发器34.48. usage_privileges34.49. user_mapping_options34.50. user_mappings34.51. view_column_usage34.52. view_routine_usage34.53. view_table_usage34.54. 视图V. 服务器端编程章35. 扩展SQL35.1. 扩展性是如何实现的35.2. PostgreSQL类型系统35.3. User-Defined Functions35.4. Query Language (SQL) Functions35.5. Function Overloading35.6. Function Volatility Categories35.7. Procedural Language Functions35.8. Internal Functions35.9. C-Language Functions35.10. User-Defined Aggregates35.11. User-Defined Types35.12. User-Defined Operators35.13. Operator Optimization Information35.14. Interfacing Extensions To Indexes35.15. 用C++扩展章36. 触发器36.1. 触发器行为概述36.3. 用 C 写触发器36.2. 数据改变的可视性36.4. 一个完整的例子章37. 规则系统37.1. The Query Tree37.2. 视图和规则系统37.3. 在INSERT，UPDATE和DELETE上的规则37.4. 规则和权限37.5. 规则和命令状态37.6. 规则与触发器得比较章38. Procedural Languages38.1. Installing Procedural Languages章39. PL/pgSQL - SQL过程语言39.1. 概述39.2. PL/pgSQL的结构39.3. 声明39.4. 表达式39.5. 基本语句39.6. 控制结构39.7. 游标39.8. 错误和消息39.9. 触发器过程39.10. PL/pgSQL Under the Hood39.11. 开发PL/pgSQL的一些提示39.12. 从OraclePL/SQL 进行移植章40. PL/Tcl - Tcl Procedural Language40.1. Overview40.2. PL/Tcl Functions and Arguments40.3. Data Values in PL/Tcl40.4. Global Data in PL/Tcl40.5. Database Access from PL/Tcl40.6. Trigger Procedures in PL/Tcl40.7. Modules and the unknown command40.8. Tcl Procedure Names章41. PL/Perl - Perl Procedural Language41.1. PL/Perl Functions and Arguments41.2. Data Values in PL/Perl41.3. Built-in Functions41.4. Global Values in PL/Perl41.6. PL/Perl Triggers41.5. Trusted and Untrusted PL/Perl41.7. PL/Perl Under the Hood章42. PL/Python - Python Procedural Language42.1. Python 2 vs. Python 342.2. PL/Python Functions42.3. Data Values42.4. Sharing Data42.5. Anonymous Code Blocks42.6. Trigger Functions42.7. Database Access42.8. Utility Functions42.9. Environment Variables章43. Server Programming Interface43.1. Interface FunctionsSpi-spi-connectSpi-spi-finishSpi-spi-pushSpi-spi-popSpi-spi-executeSpi-spi-execSpi-spi-execute-with-argsSpi-spi-prepareSpi-spi-prepare-cursorSpi-spi-prepare-paramsSpi-spi-getargcountSpi-spi-getargtypeidSpi-spi-is-cursor-planSpi-spi-execute-planSpi-spi-execute-plan-with-paramlistSpi-spi-execpSpi-spi-cursor-openSpi-spi-cursor-open-with-argsSpi-spi-cursor-open-with-paramlistSpi-spi-cursor-findSpi-spi-cursor-fetchSpi-spi-cursor-moveSpi-spi-scroll-cursor-fetchSpi-spi-scroll-cursor-moveSpi-spi-cursor-closeSpi-spi-saveplan43.2. Interface Support FunctionsSpi-spi-fnameSpi-spi-fnumberSpi-spi-getvalueSpi-spi-getbinvalSpi-spi-gettypeSpi-spi-gettypeidSpi-spi-getrelnameSpi-spi-getnspname43.3. Memory ManagementSpi-spi-pallocSpi-reallocSpi-spi-pfreeSpi-spi-copytupleSpi-spi-returntupleSpi-spi-modifytupleSpi-spi-freetupleSpi-spi-freetupletableSpi-spi-freeplan43.4. Visibility of Data Changes43.5. ExamplesVI. 参考手册I. SQL命令Sql-abortSql-alteraggregateSql-alterconversionSql-alterdatabaseSql-alterdefaultprivilegesSql-alterdomainSql-alterforeigndatawrapperSql-alterfunctionSql-altergroupSql-alterindexSql-alterlanguageSql-alterlargeobjectSql-alteroperatorSql-alteropclassSql-alteropfamilySql-alterroleSql-alterschemaSql-altersequenceSql-alterserverSql-altertableSql-altertablespaceSql-altertsconfigSql-altertsdictionarySql-altertsparserSql-altertstemplateSql-altertriggerSql-altertypeSql-alteruserSql-alterusermappingSql-alterviewSql-analyzeSql-beginSql-checkpointSql-closeSql-clusterSql-commentSql-commitSql-commit-preparedSql-copySql-createaggregateSql-createcastSql-createconstraintSql-createconversionSql-createdatabaseSql-createdomainSql-createforeigndatawrapperSql-createfunctionSql-creategroupSql-createindexSql-createlanguageSql-createoperatorSql-createopclassSql-createopfamilySql-createroleSql-createruleSql-createschemaSql-createsequenceSql-createserverSql-createtableSql-createtableasSql-createtablespaceSql-createtsconfigSql-createtsdictionarySql-createtsparserSql-createtstemplateSql-createtriggerSql-createtypeSql-createuserSql-createusermappingSql-createviewSql-deallocateSql-declareSql-deleteSql-discardSql-doSql-dropaggregateSql-dropcastSql-dropconversionSql-dropdatabaseSql-dropdomainSql-dropforeigndatawrapperSql-dropfunctionSql-dropgroupSql-dropindexSql-droplanguageSql-dropoperatorSql-dropopclassSql-dropopfamilySql-drop-ownedSql-droproleSql-dropruleSql-dropschemaSql-dropsequenceSql-dropserverSql-droptableSql-droptablespaceSql-droptsconfigSql-droptsdictionarySql-droptsparserSql-droptstemplateSql-droptriggerSql-droptypeSql-dropuserSql-dropusermappingSql-dropviewSql-endSql-executeSql-explainSql-fetchSql-grantSql-insertSql-listenSql-loadSql-lockSql-moveSql-notifySql-prepareSql-prepare-transactionSql-reassign-ownedSql-reindexSql-release-savepointSql-resetSql-revokeSql-rollbackSql-rollback-preparedSql-rollback-toSql-savepointSql-selectSql-selectintoSql-setSql-set-constraintsSql-set-roleSql-set-session-authorizationSql-set-transactionSql-showSql-start-transactionSql-truncateSql-unlistenSql-updateSql-vacuumSql-valuesII. 客户端应用程序App-clusterdbApp-createdbApp-createlangApp-createuserApp-dropdbApp-droplangApp-dropuserApp-ecpgApp-pgconfigApp-pgdumpApp-pg-dumpallApp-pgrestoreApp-psqlApp-reindexdbApp-vacuumdbIII. PostgreSQL服务器应用程序App-initdbApp-pgcontroldataApp-pg-ctlApp-pgresetxlogApp-postgresApp-postmasterVII. 内部章44. PostgreSQL内部概览44.1. 查询路径44.2. 连接是如何建立起来的44.3. 分析器阶段44.4. ThePostgreSQL规则系统44.5. 规划器/优化器44.6. 执行器章45. 系统表45.1. 概述45.2. pg_aggregate45.3. pg_am45.4. pg_amop45.5. pg_amproc45.6. pg_attrdef45.7. pg_attribute45.8. pg_authid45.9. pg_auth_members45.10. pg_cast45.11. pg_class45.12. pg_constraint45.13. pg_conversion45.14. pg_database45.15. pg_db_role_setting45.16. pg_default_acl45.17. pg_depend45.18. pg_description45.19. pg_enum45.20. pg_foreign_data_wrapper45.21. pg_foreign_server45.22. pg_index45.23. pg_inherits45.24. pg_language45.25. pg_largeobject45.26. pg_largeobject_metadata45.27. pg_namespace45.28. pg_opclass45.29. pg_operator45.30. pg_opfamily45.31. pg_pltemplate45.32. pg_proc45.33. pg_rewrite45.34. pg_shdepend45.35. pg_shdescription45.36. pg_statistic45.37. pg_tablespace45.38. pg_trigger45.39. pg_ts_config45.40. pg_ts_config_map45.41. pg_ts_dict45.42. pg_ts_parser45.43. pg_ts_template45.44. pg_type45.45. pg_user_mapping45.46. System Views45.47. pg_cursors45.48. pg_group45.49. pg_indexes45.50. pg_locks45.51. pg_prepared_statements45.52. pg_prepared_xacts45.53. pg_roles45.54. pg_rules45.55. pg_settings45.56. pg_shadow45.57. pg_stats45.58. pg_tables45.59. pg_timezone_abbrevs45.60. pg_timezone_names45.61. pg_user45.62. pg_user_mappings45.63. pg_views章46. Frontend/Backend Protocol46.1. Overview46.2. Message Flow46.3. Streaming Replication Protocol46.4. Message Data Types46.5. Message Formats46.6. Error and Notice Message Fields46.7. Summary of Changes since Protocol 2.047. PostgreSQL Coding Conventions47.1. Formatting47.2. Reporting Errors Within the Server47.3. Error Message Style Guide章48. Native Language Support48.1. For the Translator48.2. For the Programmer章49. Writing A Procedural Language Handler章50. Genetic Query Optimizer50.1. Query Handling as a Complex Optimization Problem50.2. Genetic Algorithms50.3. Genetic Query Optimization (GEQO) in PostgreSQL50.4. Further Reading章51. 索引访问方法接口定义51.1. 索引的系统表记录51.2. 索引访问方法函数51.3. 索引扫描51.4. 索引锁的考量51.5. 索引唯一性检查51.6. 索引开销估计函数章52. GiST Indexes52.1. Introduction52.2. Extensibility52.3. Implementation52.4. Examples52.5. Crash Recovery章53. GIN Indexes53.1. Introduction53.2. Extensibility53.3. Implementation53.4. GIN tips and tricks53.5. Limitations53.6. Examples章54. 数据库物理存储54.1. 数据库文件布局54.2. TOAST54.3. 自由空间映射54.4. 可见映射54.5. 数据库分页文件章55. BKI后端接口55.1. BKI 文件格式55.2. BKI命令55.3. 系统初始化的BKI文件的结构55.4. 例子章56. 规划器如何使用统计信息56.1. 行预期的例子VIII. 附录A. PostgreSQL错误代码B. 日期/时间支持B.1. 日期/时间输入解析B.2. 日期/时间关键字B.3. 日期/时间配置文件B.4. 日期单位的历史C. SQL关键字D. SQL ConformanceD.1. Supported FeaturesD.2. Unsupported FeaturesE. Release NotesRelease-0-01Release-0-02Release-0-03Release-1-0Release-1-01Release-1-02Release-1-09Release-6-0Release-6-1Release-6-1-1Release-6-2Release-6-2-1Release-6-3Release-6-3-1Release-6-3-2Release-6-4Release-6-4-1Release-6-4-2Release-6-5Release-6-5-1Release-6-5-2Release-6-5-3Release-7-0Release-7-0-1Release-7-0-2Release-7-0-3Release-7-1Release-7-1-1Release-7-1-2Release-7-1-3Release-7-2Release-7-2-1Release-7-2-2Release-7-2-3Release-7-2-4Release-7-2-5Release-7-2-6Release-7-2-7Release-7-2-8Release-7-3Release-7-3-1Release-7-3-10Release-7-3-11Release-7-3-12Release-7-3-13Release-7-3-14Release-7-3-15Release-7-3-16Release-7-3-17Release-7-3-18Release-7-3-19Release-7-3-2Release-7-3-20Release-7-3-21Release-7-3-3Release-7-3-4Release-7-3-5Release-7-3-6Release-7-3-7Release-7-3-8Release-7-3-9Release-7-4Release-7-4-1Release-7-4-10Release-7-4-11Release-7-4-12Release-7-4-13Release-7-4-14Release-7-4-15Release-7-4-16Release-7-4-17Release-7-4-18Release-7-4-19Release-7-4-2Release-7-4-20Release-7-4-21Release-7-4-22Release-7-4-23Release-7-4-24Release-7-4-25Release-7-4-26Release-7-4-27Release-7-4-28Release-7-4-29Release-7-4-3Release-7-4-30Release-7-4-4Release-7-4-5Release-7-4-6Release-7-4-7Release-7-4-8Release-7-4-9Release-8-0Release-8-0-1Release-8-0-10Release-8-0-11Release-8-0-12Release-8-0-13Release-8-0-14Release-8-0-15Release-8-0-16Release-8-0-17Release-8-0-18Release-8-0-19Release-8-0-2Release-8-0-20Release-8-0-21Release-8-0-22Release-8-0-23Release-8-0-24Release-8-0-25Release-8-0-26Release-8-0-3Release-8-0-4Release-8-0-5Release-8-0-6Release-8-0-7Release-8-0-8Release-8-0-9Release-8-1Release-8-1-1Release-8-1-10Release-8-1-11Release-8-1-12Release-8-1-13Release-8-1-14Release-8-1-15Release-8-1-16Release-8-1-17Release-8-1-18Release-8-1-19Release-8-1-2Release-8-1-20Release-8-1-21Release-8-1-22Release-8-1-23Release-8-1-3Release-8-1-4Release-8-1-5Release-8-1-6Release-8-1-7Release-8-1-8Release-8-1-9Release-8-2Release-8-2-1Release-8-2-10Release-8-2-11Release-8-2-12Release-8-2-13Release-8-2-14Release-8-2-15Release-8-2-16Release-8-2-17Release-8-2-18Release-8-2-19Release-8-2-2Release-8-2-20Release-8-2-21Release-8-2-3Release-8-2-4Release-8-2-5Release-8-2-6Release-8-2-7Release-8-2-8Release-8-2-9Release-8-3Release-8-3-1Release-8-3-10Release-8-3-11Release-8-3-12Release-8-3-13Release-8-3-14Release-8-3-15Release-8-3-2Release-8-3-3Release-8-3-4Release-8-3-5Release-8-3-6Release-8-3-7Release-8-3-8Release-8-3-9Release-8-4Release-8-4-1Release-8-4-2Release-8-4-3Release-8-4-4Release-8-4-5Release-8-4-6Release-8-4-7Release-8-4-8Release-9-0Release-9-0-1Release-9-0-2Release-9-0-3Release-9-0-4F. 额外提供的模块F.1. adminpackF.2. auto_explainF.3. btree_ginF.4. btree_gistF.5. chkpassF.6. citextF.7. cubeF.8. dblinkContrib-dblink-connectContrib-dblink-connect-uContrib-dblink-disconnectContrib-dblinkContrib-dblink-execContrib-dblink-openContrib-dblink-fetchContrib-dblink-closeContrib-dblink-get-connectionsContrib-dblink-error-messageContrib-dblink-send-queryContrib-dblink-is-busyContrib-dblink-get-notifyContrib-dblink-get-resultContrib-dblink-cancel-queryContrib-dblink-get-pkeyContrib-dblink-build-sql-insertContrib-dblink-build-sql-deleteContrib-dblink-build-sql-updateF.9. dict_intF.10. dict_xsynF.11. earthdistanceF.12. fuzzystrmatchF.13. hstoreF.14. intaggF.15. intarrayF.16. isnF.17. loF.18. ltreeF.19. oid2nameF.20. pageinspectF.21. passwordcheckF.22. pg_archivecleanupF.23. pgbenchF.24. pg_buffercacheF.25. pgcryptoF.26. pg_freespacemapF.27. pgrowlocksF.28. pg_standbyF.29. pg_stat_statementsF.30. pgstattupleF.31. pg_trgmF.32. pg_upgradeF.33. segF.34. spiF.35. sslinfoF.36. tablefuncF.37. test_parserF.38. tsearch2F.39. unaccentF.40. uuid-osspF.41. vacuumloF.42. xml2G. 外部项目G.1. 客户端接口G.2. 过程语言G.3. 扩展H. The Source Code RepositoryH.1. Getting The Source Via GitI. 文档I.1. DocBookI.2. 工具集I.3. 制作文档I.4. 文档写作I.5. 风格指导J. 首字母缩略词参考书目BookindexIndex

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45.2. pg_aggregate

pg_aggregate存储与聚集函数有关的信息。聚集函数是对一个数值集 (通常每个匹配查询条件的行中的一个字段)进行操作的函数，它返回从这些值中计算出的一个数值。 典型的聚集函数是 sum, count, 和 max 。pg_aggregate里的每条记录都是一条 pg_proc里面 的记录的扩展。pg_proc记录承载该聚集的名字、输入和输出数据类型， 以及其它一些和普通函数类似的信息。

Table 45-2. pg_aggregate Columns

名称 类型 应用 描述
aggfnoid regproc pg_proc.oid 聚合函数的 pg_proc OID
aggtransfn regproc pg_proc.oid 转换函数
aggfinalfn regproc pg_proc.oid 最终处理函数（如果没有则为零）
aggsortop oid pg_operator.oid 关联排序操作符(零或者无)
aggtranstype oid pg_type.oid 此聚集函数的内部转换(状态)数据的数据类型
agginitval text   转换状态的初始值。这是一个文本字段，它包含初始值的外部字符串表现形式。 如果这个字段是null，那么转换状态值从null开始。

新聚合函数是用 CREATE AGGREGATE命令注册的。 参阅Section 35.10获取关于编写聚合函数以及转换函数的含义等的更多信息。

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