【OpenGauss源码学习 —— （ALTER TABLE（Add Column））】

2024-04-29 02:28
文章标签 源码 学习 table opengauss column add alter

本文主要是介绍【OpenGauss源码学习 —— （ALTER TABLE（Add Column））】，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

ALTER TABLE（Add Column）

  • 概述
  • ATController 函数
  • ATPrepCmd 函数
  • ATRewriteCatalogs
  • ATExecCmd 函数
  • ATExecAddColumn 函数
  • ATRewriteTables 函数

声明：本文的部分内容参考了他人的文章。在编写过程中，我们尊重他人的知识产权和学术成果，力求遵循合理使用原则，并在适用的情况下注明引用来源。
本文主要参考了 OpenGauss5.1.0 的开源代码和《OpenGauss数据库源码解析》一书

概述

  ALTER TABLE 命令中的 Add Column 子命令用于向已存在的数据库表中添加新的列。这个过程涉及到更新表的元数据，以及可能对表的物理结构进行调整，特别是在需要初始化新列的默认值时。在数据库系统中，执行 Add Column 操作通常需要考虑锁定机制以避免与其他数据库操作冲突，并且必须确保数据的完整性和一致性不被破坏。此外，添加列的操作还可能触发相关的事件触发器，这些触发器可以用于执行一些定制的逻辑，如自动更新相关视图或维护日志等。整个过程需要高效地管理，以最小化对数据库性能的影响，特别是在涉及大量数据或高并发环境下。

ATController 函数

  ATController 函数属于 openGauss 数据库系统中处理 ALTER TABLE 语句的核心控制逻辑。此函数主要负责协调 ALTER TABLE 操作的各个阶段，包括检查命令、准备命令、更新系统目录、分布式系统的特定处理以及实际的表重写。
  这个函数主要负责处理 ALTER TABLE 命令的执行，涵盖了从命令检查、预处理，到系统目录更新和表的实际修改。在分布式环境中（如 PGXC，PostgreSQL-XC），还处理数据的重新分配，确保在分布式节点间数据一致性和表结构的同步。通过多阶段的设计，确保了操作的原子性和一致性，同时通过精细的错误检查和条件处理，保持了系统的健壮性。函数源码如下所示（路径：src\gausskernel\optimizer\commands\tablecmds.cpp）

// 定义函数ATController，参数包括解析树、关系、命令列表、是否递归和锁模式
static void ATController(AlterTableStmt *parsetree, Relation rel, List* cmds, bool recurse, LOCKMODE lockmode)
{List* wqueue = NIL; // 工作队列，用于存储处理过程中的临时信息ListCell* lcmd = NULL; // 用于遍历命令列表的循环变量#ifdef PGXCRedistribState* redistribState = NULL; // 分布式状态，用于处理数据重新分配bool doRedistribute = false; // 标记是否需要进行数据重新分配
#endif// 第一阶段：对命令进行初步检查，创建工作队列foreach (lcmd, cmds) {AlterTableCmd* cmd = (AlterTableCmd*)lfirst(lcmd); // 从命令列表中取出命令#ifdef PGXC// 在集群中检查ALTER TABLE的限制ATCheckCmd(rel, cmd);
#endif// 检查账本表命令的兼容性ATCheckLedgerTableCmd(rel, cmd);// 准备命令，包括验证和设置工作队列ATPrepCmd(&wqueue, rel, cmd, recurse, false, lockmode);}#ifdef PGXC// 仅在本地协调器上检查if (IS_PGXC_COORDINATOR) {ListCell* ltab = NULL;// 对于父表进行数据重新分配处理，不适用于子表或递归情况foreach (ltab, wqueue) {AlteredTableInfo* tab = (AlteredTableInfo*)lfirst(ltab);// 如果当前关系是处理队列中的关系且有重新分配命令if (RelationGetRelid(rel) == tab->relid && list_length(tab->subcmds[AT_PASS_DISTRIB]) > 0) {// 检查是否有与重新分配不兼容的命令doRedistribute = true;if (!IsConnFromCoord()) {if (list_length(tab->subcmds[AT_PASS_ADD_COL]) > 0 || list_length(tab->subcmds[AT_PASS_DROP]) > 0 ||list_length(tab->subcmds[AT_PASS_ALTER_TYPE]) > 0 ||list_length(tab->subcmds[AT_PASS_OLD_CONSTR]) > 0 ||list_length(tab->subcmds[AT_PASS_COL_ATTRS]) > 0 ||list_length(tab->subcmds[AT_PASS_ADD_INDEX]) > 0 ||list_length(tab->subcmds[AT_PASS_ADD_CONSTR]) > 0 ||list_length(tab->subcmds[AT_PASS_MISC]) > 0)ereport(ERROR, // 报错，操作不兼容(errcode(ERRCODE_STATEMENT_TOO_COMPLEX),errmsg("Incompatible operation with data redistribution")));// 扫描重新分配命令并优化操作redistribState = BuildRedistribCommands(RelationGetRelid(rel), tab->subcmds[AT_PASS_DISTRIB]);}break;}}}
#endif// 关闭关系，但直到提交前保持锁relation_close(rel, NoLock);// 第二阶段：更新系统目录ATRewriteCatalogs(&wqueue, lockmode);#ifdef PGXC// 无效缓存以重新分配关系if (doRedistribute) {Relation rel2 = relation_open(RelationGetRelid(rel), NoLock);// 使与此关系相关的所有条目无效CacheInvalidateRelcache(rel2);// 确保重新构建定位信息RelationCacheInvalidateEntry(RelationGetRelid(rel));relation_close(rel2, NoLock);}// 如果有重新分配状态，则释放它if (redistribState != NULL)FreeRedistribState(redistribState);
#endif// 第三阶段：扫描/重写表ATRewriteTables(parsetree, &wqueue, lockmode);
}

ATPrepCmd 函数

  函数 ATPrepCmd 是用于处理 ALTER TABLE 命令在执行前的预处理的关键函数。这个函数作为“交通警察”（Traffic cop）角色，负责第一阶段的操作，包括简单的递归处理、权限检查、以及准备命令的进一步执行。它通过分析传入的 AlterTableCmd 命令来决定如何处理各种 ALTER TABLE 子命令，如添加列、添加分区、修改列类型等，并将这些命令根据类型分配到不同的处理队列中。
  具体来说，这个函数首先为每个表创建或查找一个工作队列条目，复制原始子命令以避免在处理不同子表时出现解析冲突。然后，根据子命令的类型，执行相应的权限检查，并根据需要对子表进行递归处理。对于每种类型的命令，函数会调用相应的预处理函数（如 ATPrepAddColumn、ATPrepDropColumn 等），并根据命令的性质决定是否在执行阶段进行递归。最后，根据命令的具体类型，将命令添加到工作队列的相应部分，以便在第二阶段进行系统目录的更新和实际的表更改。函数源码如下所示（路径：src\gausskernel\optimizer\commands\tablecmds.cpp）

/** ATPrepCmd** Traffic cop for ALTER TABLE Phase 1 operations, including simple* recursion and permission checks.** Caller must have acquired appropriate lock type on relation already.* This lock should be held until commit.*/
static void ATPrepCmd(List** wqueue, Relation rel, AlterTableCmd* cmd, bool recurse, bool recursing, LOCKMODE lockmode,bool isDeltaTable)
{AlteredTableInfo* tab = NULL;int pass;/* Find or create work queue entry for this table */tab = ATGetQueueEntry(wqueue, rel, isDeltaTable);/** Copy the original subcommand for each table.  This avoids conflicts* when different child tables need to make different parse* transformations (for example, the same column may have different column* numbers in different children).*/cmd = (AlterTableCmd*)copyObject(cmd);/** Do permissions checking, recursion to child tables if needed, and any* additional phase-1 processing needed.*/switch (cmd->subtype) {case AT_AddColumn: /* ADD COLUMN */ATSimplePermissions(rel, ATT_TABLE | ATT_COMPOSITE_TYPE | ATT_FOREIGN_TABLE | ATT_SEQUENCE);ATPrepAddColumn(wqueue, tab, rel, recurse, recursing, cmd, lockmode);/* Recursion occurs during execution phase */pass = AT_PASS_ADD_COL;break;case AT_AddPartition: /* ADD PARTITION */ATSimplePermissions(rel, ATT_TABLE);ATPrepAddPartition(rel);/* Recursion occurs during execution phase */pass = AT_PASS_ADD_PARTITION;break;case AT_AddSubPartition: /* ADD SUBPARTITION */ATSimplePermissions(rel, ATT_TABLE);ATPrepAddSubPartition(rel);/* ADD SUBPARTITION obeys the same recursion order with ADD PARTITION */pass = AT_PASS_ADD_PARTITION;break;case AT_AddColumnToView: /* add column via CREATE OR REPLACE* VIEW */ATSimplePermissions(rel, ATT_VIEW);ATPrepAddColumn(wqueue, NULL, rel, recurse, recursing, cmd, lockmode);/* Recursion occurs during execution phase */pass = AT_PASS_ADD_COL;break;case AT_ColumnDefault: /* ALTER COLUMN DEFAULT *//** We allow defaults on views so that INSERT into a view can have* default-ish behavior.  This works because the rewriter* substitutes default values into INSERTs before it expands* rules.*/ATSimplePermissions(rel, ATT_TABLE | ATT_VIEW);ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode);ATPrepCheckDefault(cmd->def);/* No command-specific prep needed */pass = cmd->def ? AT_PASS_ADD_CONSTR : AT_PASS_DROP;break;case AT_DropNotNull: /* ALTER COLUMN DROP NOT NULL */ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode);/* No command-specific prep needed */pass = AT_PASS_DROP;ATCheckNotNullConstr(cmd, tab);break;case AT_SetNotNull: /* ALTER COLUMN SET NOT NULL */ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode);/* No command-specific prep needed */pass = AT_PASS_ADD_CONSTR;ATCheckNotNullConstr(cmd, tab);break;case AT_SetStatistics: /* ALTER COLUMN SET STATISTICS */ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode);/* Performs own permission checks */ATPrepSetStatistics(rel);pass = AT_PASS_MISC;break;case AT_AddStatistics:    /* ADD STATISTICS */case AT_DeleteStatistics: /* DELETE STATISTICS */ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode);/* Performs own permission checks */ATPrepSetStatistics(rel);es_check_alter_table_statistics(rel, cmd);pass = AT_PASS_MISC;break;case AT_SetOptions:   /* ALTER COLUMN SET ( options ) */case AT_ResetOptions: /* ALTER COLUMN RESET ( options ) */ATSimplePermissions(rel, ATT_TABLE | ATT_MATVIEW | ATT_INDEX | ATT_FOREIGN_TABLE);/* This command never recurses */pass = AT_PASS_MISC;break;case AT_SetStorage: /* ALTER COLUMN SET STORAGE */ATSimplePermissions(rel, ATT_TABLE | ATT_MATVIEW);ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode);/* No command-specific prep needed */pass = AT_PASS_MISC;break;case AT_DropColumn: /* DROP COLUMN */ATSimplePermissions(rel,ATT_TABLE | ATT_COMPOSITE_TYPE | ATT_FOREIGN_TABLE |(u_sess->attr.attr_common.IsInplaceUpgrade ? ATT_VIEW : ATT_NULL));ATPrepDropColumn(wqueue, rel, recurse, recursing, cmd, lockmode);/* Recursion occurs during execution phase */pass = AT_PASS_DROP;break;case AT_DropPartition: /* DROP PARTITION */ATSimplePermissions(rel, ATT_TABLE);ATPrepDropPartition(rel);/* Recursion occurs during execution phase */pass = AT_PASS_DROP;break;case AT_DropSubPartition: /* DROP SUBPARTITION */ATSimplePermissions(rel, ATT_TABLE);ATPrepDropSubPartition(rel);/* Recursion occurs during execution phase */pass = AT_PASS_DROP;break;case AT_UnusableIndexPartition: /* UNUSEABLE INDEX PARTITION */ATSimplePermissions(rel, ATT_INDEX);ATPrepUnusableIndexPartition(rel);/* Recursion occurs during execution phase */pass = AT_PASS_MISC;break;case AT_UnusableAllIndexOnPartition: /* UNUSEABLE ALL INDEX ON PARTITION */ATSimplePermissions(rel, ATT_TABLE | ATT_INDEX);ATPrepUnusableAllIndexOnPartition(rel);/* Recursion occurs during execution phase */pass = AT_PASS_MISC;break;case AT_AddIndex: /* ADD INDEX */ATSimplePermissions(rel, ATT_TABLE | ATT_MATVIEW | ATT_FOREIGN_TABLE);/* This command never recurses *//* No command-specific prep needed */pass = AT_PASS_ADD_INDEX;break;case AT_AddConstraint: /* ADD CONSTRAINT */ATSimplePermissions(rel, ATT_TABLE);/* Recursion occurs during execution phase *//* No command-specific prep needed except saving recurse flag */if (recurse)cmd->subtype = AT_AddConstraintRecurse;pass = AT_PASS_ADD_CONSTR;break;case AT_AddIndexConstraint: /* ADD CONSTRAINT USING INDEX */ATSimplePermissions(rel, ATT_TABLE);/* This command never recurses *//* No command-specific prep needed */pass = AT_PASS_ADD_CONSTR;break;case AT_DropConstraint: /* DROP CONSTRAINT *//* @hdfs* ATSimplePermissions's second parameter is change from ATT_TABLE to* ATT_TABLE|ATT_FOREIGN_TABLE to suppert droping HDFS foreign table.*/ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);/* Recursion occurs during execution phase *//* No command-specific prep needed except saving recurse flag */if (recurse)cmd->subtype = AT_DropConstraintRecurse;pass = AT_PASS_DROP;break;case AT_AlterColumnType: /* ALTER COLUMN TYPE */ATSimplePermissions(rel, ATT_TABLE | ATT_COMPOSITE_TYPE | ATT_FOREIGN_TABLE);/* Performs own recursion */ATPrepAlterColumnType(wqueue, tab, rel, recurse, recursing, cmd, lockmode);pass = AT_PASS_ALTER_TYPE;ATCheckDuplicateColumn(cmd, tab->subcmds[pass]);break;case AT_AlterColumnGenericOptions:ATSimplePermissions(rel, ATT_FOREIGN_TABLE);/* This command never recurses *//* No command-specific prep needed */pass = AT_PASS_MISC;break;case AT_ChangeOwner: /* ALTER OWNER *//* This command never recurses *//* No command-specific prep needed */pass = AT_PASS_MISC;break;case AT_ClusterOn:   /* CLUSTER ON */case AT_DropCluster: /* SET WITHOUT CLUSTER */ATSimplePermissions(rel, ATT_TABLE | ATT_MATVIEW);/* These commands never recurse *//* No command-specific prep needed */pass = AT_PASS_MISC;break;case AT_AddOids: /* SET WITH OIDS *//** partitioned table can not be setted with or without oids*/if (RELATION_IS_PARTITIONED(rel)) {ereport(ERROR,(errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("cannot set with oids on partitioned table")));}ATSimplePermissions(rel, ATT_TABLE);if (!rel->rd_rel->relhasoids || recursing)ATPrepAddOids(wqueue, rel, recurse, cmd, lockmode);/* Recursion occurs during execution phase */pass = AT_PASS_ADD_COL;break;case AT_DropOids: /* SET WITHOUT OIDS *//** partitioned table can not be setted with or without oids*/if (RELATION_IS_PARTITIONED(rel)) {ereport(ERROR,(errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("cannot set without oids on partitioned table")));}ATSimplePermissions(rel, ATT_TABLE);/* Performs own recursion */if (rel->rd_rel->relhasoids) {AlterTableCmd* dropCmd = makeNode(AlterTableCmd);dropCmd->subtype = AT_DropColumn;dropCmd->name = pstrdup("oid");dropCmd->behavior = cmd->behavior;ATPrepCmd(wqueue, rel, dropCmd, recurse, false, lockmode);}pass = AT_PASS_DROP;break;case AT_SetTableSpace: /* SET TABLESPACE */case AT_SetPartitionTableSpace:ATSimplePermissions(rel, ATT_TABLE | ATT_INDEX | ATT_MATVIEW);/* This command never recurses */ATPrepSetTableSpace(tab, rel, cmd->name, lockmode);pass = AT_PASS_MISC; /* doesn't actually matter */break;case AT_UnusableIndex:case AT_SetRelOptions:     /* SET (...) */case AT_ResetRelOptions:   /* RESET (...) */case AT_ReplaceRelOptions: /* reset them all, then set just these */case AT_InvisibleIndex:case AT_VisibleIndex:ATSimplePermissions(rel, ATT_TABLE | ATT_INDEX | ATT_VIEW);/* This command never recurses *//* No command-specific prep needed */pass = AT_PASS_MISC;break;case AT_AddInherit: /* INHERIT */ATSimplePermissions(rel, ATT_TABLE);/* This command never recurses */ATPrepAddInherit(rel);pass = AT_PASS_MISC;break;case AT_ValidateConstraint: /* VALIDATE CONSTRAINT */ATSimplePermissions(rel, ATT_TABLE);/* Recursion occurs during execution phase *//* No command-specific prep needed except saving recurse flag */if (recurse)cmd->subtype = AT_ValidateConstraintRecurse;pass = AT_PASS_MISC;break;case AT_ReplicaIdentity: /* REPLICA IDENTITY ... */ATSimplePermissions(rel, ATT_TABLE);pass = AT_PASS_MISC;/* This command never recurses *//* No command-specific prep needed */break;case AT_EnableTrig: /* ENABLE TRIGGER variants */case AT_EnableAlwaysTrig:case AT_EnableReplicaTrig:case AT_EnableTrigAll:case AT_EnableTrigUser:case AT_DisableTrig: /* DISABLE TRIGGER variants */case AT_DisableTrigAll:case AT_DisableTrigUser:case AT_EnableRule: /* ENABLE/DISABLE RULE variants */case AT_EnableAlwaysRule:case AT_EnableReplicaRule:case AT_DisableRule:case AT_EnableRls: /* ENABLE/DISABLE ROW LEVEL SECURITY */case AT_DisableRls:case AT_ForceRls: /* FORCE/NO-FORCE ROW LEVEL SECURITY */case AT_NoForceRls:case AT_EncryptionKeyRotation:case AT_DropInherit: /* NO INHERIT */case AT_AddOf:       /* OF */case AT_DropOf:      /* NOT OF */case AT_SetAutoIncrement:case AT_SetCharsetCollate:ATSimplePermissions(rel, ATT_TABLE);/* These commands never recurse *//* No command-specific prep needed */pass = AT_PASS_MISC;break;case AT_ConvertCharset:ATSimplePermissions(rel, ATT_TABLE);sqlcmd_alter_prep_convert_charset(tab, rel, cmd, lockmode);pass = AT_PASS_MISC;break;case AT_GenericOptions:ATSimplePermissions(rel, ATT_FOREIGN_TABLE);/* No command-specific prep needed */pass = AT_PASS_MISC;break;case AT_SET_COMPRESS:ATSimplePermissions(rel, ATT_TABLE);pass = AT_PASS_MISC;break;case AT_EnableRowMoveMent:ATSimplePermissions(rel, ATT_TABLE);ATPrepEnableRowMovement(rel);pass = AT_PASS_MISC;break;case AT_DisableRowMoveMent:ATSimplePermissions(rel, ATT_TABLE);ATPrepDisableRowMovement(rel);pass = AT_PASS_MISC;break;case AT_TruncatePartition:ATPrepTruncatePartition(rel);pass = AT_PASS_MISC;break;case AT_TruncateSubPartition:ATPrepTruncateSubPartition(rel);pass = AT_PASS_MISC;break;case AT_ExchangePartition:ATSimplePermissions(rel, ATT_TABLE);ATPrepExchangePartition(rel);pass = AT_PASS_MISC;break;case AT_MergePartition:ATSimplePermissions(rel, ATT_TABLE);ATPrepMergePartition(rel);pass = AT_PASS_MISC;break;case AT_SplitPartition:ATSimplePermissions(rel, ATT_TABLE);ATPrepSplitPartition(rel);pass = AT_PASS_MISC;break;case AT_AddIntoCBI:ATSimplePermissions(rel, ATT_INDEX);pass = AT_PASS_MISC;break;case AT_SplitSubPartition:ATSimplePermissions(rel, ATT_TABLE);ATPrepSplitSubPartition(rel);pass = AT_PASS_MISC;break;case AT_ResetPartitionno:ATSimplePermissions(rel, ATT_TABLE);ATPrepResetPartitionno(rel);pass = AT_PASS_MISC;break;case AT_ModifyColumn:ATSimplePermissions(rel, ATT_TABLE);ATPrepAlterModifyColumn(wqueue, tab, rel, recurse, recursing, cmd, lockmode);pass = AT_PASS_ALTER_TYPE;ATAlterCheckModifiyColumnRepeatedly(cmd, tab->subcmds[pass]);break;
#ifdef PGXCcase AT_DistributeBy:case AT_SubCluster:ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE | ATT_MATVIEW);/* No command-specific prep needed */pass = AT_PASS_DISTRIB;break;/* @hdfs* The HDFS foreign table support 'ALTER FOREIGN TABLE ADD NODE/DELETE NODE' cmd.*/case AT_AddNodeList:case AT_DeleteNodeList:case AT_UpdateSliceLike:ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);/* No command-specific prep needed */pass = AT_PASS_DISTRIB;break;case AT_COMMENTS:pass = AT_COMMENT;break;
#endifdefault: /* oops */ereport(ERROR,(errcode(ERRCODE_UNRECOGNIZED_NODE_TYPE),errmsg("unrecognized alter table type: %d", (int)cmd->subtype)));pass = 0; /* keep compiler quiet */break;}/* Add the subcommand to the appropriate list for phase 2 */tab->subcmds[pass] = lappend(tab->subcmds[pass], cmd);
}

ATRewriteCatalogs

  emspATRewriteCatalogs 函数用于处理 ALTER TABLE 命令的第二阶段操作，即实际执行修改数据库目录的操作。该函数负责将 ALTER TABLE 命令中的各种子命令按照安全的顺序执行，如添加、修改列类型、删除等。通过并行处理每个表的不同修改步骤，确保了修改过程中数据一致性和执行效率。此外，还处理了特殊情况如自增索引的检查和 TOAST 表的创建，确保表的结构和数据完整性。函数源码如下所示（路径：src\gausskernel\optimizer\commands\tablecmds.cpp）

static void ATRewriteCatalogs(List** wqueue, LOCKMODE lockmode)
{int pass;ListCell* ltab = NULL;// 以多个阶段进行处理，确保可以正确处理依赖和并发问题for (pass = 0; pass < AT_NUM_PASSES; pass++) {// 遍历待处理的表列表foreach (ltab, *wqueue) {AlteredTableInfo* tab = (AlteredTableInfo*)lfirst(ltab); // 从列表中取出一个表的修改信息List* subcmds = tab->subcmds[pass]; // 获取当前阶段对该表的子命令列表Relation rel;ListCell* lcmd = NULL;if (subcmds == NIL) // 如果没有子命令，则跳过当前表continue;// 打开表，但不加锁，因为锁已在之前的阶段中获取rel = relation_open(tab->relid, NoLock);// 遍历并执行所有子命令foreach (lcmd, subcmds)ATExecCmd(wqueue, tab, rel, (AlterTableCmd*)lfirst(lcmd), lockmode);// 如果当前是处理类型更改的阶段，执行清理工作if (pass == AT_PASS_ALTER_TYPE && !tab->isDeltaTable)ATPostAlterTypeCleanup(wqueue, tab, lockmode);// 关闭对表的引用relation_close(rel, NoLock);}}// 再次遍历处理队列，检查是否需要添加 TOAST 表或其他后续处理foreach (ltab, *wqueue) {AlteredTableInfo* tab = (AlteredTableInfo*)lfirst(ltab);// 更新生成的表达式if (tab->is_first_after) {UpdateGeneratedExpr(tab);}// 如果是全局临时表，创建存储文件if (get_rel_persistence(tab->relid) == RELPERSISTENCE_GLOBAL_TEMP) {gtt_create_storage_files(tab->relid);}// 检查是否需要为常规表或物化视图创建 TOAST 表if ((tab->relkind == RELKIND_RELATION || tab->relkind == RELKIND_MATVIEW) &&!u_sess->attr.attr_sql.enable_cluster_resize) {Relation rel = relation_open(tab->relid, NoLock);Datum toast_reloptions = (Datum)0;if (rel->rd_options != NULL && RelationIsTableAccessMethodUStoreType(rel->rd_options)) {List* optsList = NIL;DefElem* def = makeDefElem(pstrdup("storage_type"),(Node*)makeString((char*)(TABLE_ACCESS_METHOD_USTORE)));optsList = lappend(optsList, def);toast_reloptions = transformRelOptions((Datum)0, optsList, NULL, NULL, false, false);}AlterTableCreateToastTable(tab->relid, toast_reloptions);relation_close(rel, NoLock);}// 检查自增索引if (tab->relkind == RELKIND_RELATION) {CheckRelAutoIncrementIndex(tab->relid, NoLock);}// 重新创建所有表触发器foreach_cell(def_item, tab->changedTriggerDefs) {char* cmd_str = (char*)lfirst(def_item);List* raw_parsetree_list = raw_parser(cmd_str);Node* stmt = (Node*)linitial(raw_parsetree_list);Assert(IsA(stmt, CreateTrigStmt)); // 确保解析结果为创建触发器语句(void)CreateTrigger((CreateTrigStmt*)stmt, cmd_str, InvalidOid, InvalidOid, InvalidOid, InvalidOid, false);}}
}

ATExecCmd 函数

  ATExecCmd 函数负责根据子命令的类型将 ALTER TABLE 命令分派到适当的执行例程。这个函数是 ALTER TABLE 操作中的关键组成部分，用于确保各种表结构变更操作能够正确执行。
  它首先会根据命令类型检查是否涉及到分区操作，并进行相应的处理。接着，它根据命令的子类型调用相应的处理函数。此外，函数还负责记录对象的修改时间、向事件触发器报告操作，以及管理命令执行过程中的锁和命令计数器，确保操作的原子性和可见性。这些都是确保数据库结构变更正确、高效执行的重要步骤。函数源码如下所示（路径：src\gausskernel\optimizer\commands\tablecmds.cpp）

// 函数定义，包括工作队列、被修改的表信息、操作的表关系、具体的修改命令和锁模式
static void ATExecCmd(List** wqueue, AlteredTableInfo* tab, Relation rel, AlterTableCmd* cmd, LOCKMODE lockmode)
{ObjectAddress address = InvalidObjectAddress; // 初始化对象地址，用于记录本次操作影响的数据库对象elog(ES_LOGLEVEL, "[ATExecCmd] cmd subtype: %d", cmd->subtype); // 记录日志，显示命令的子类型// 如果命令类型属于分区DDL命令，并且关系对象是一个分区表if (PARTITION_DDL_CMD(cmd->subtype) && RELATION_IS_PARTITIONED(rel)) {int partitionno = -GetCurrentPartitionNo(RelOidGetPartitionTupleid(rel->rd_id)); // 获取当前分区号if (!PARTITIONNO_IS_VALID(partitionno)) { // 如果分区号无效RelationResetPartitionno(rel->rd_id, ShareUpdateExclusiveLock); // 重置分区号}}// 根据命令的子类型，分派到相应的处理例程switch (cmd->subtype) {case AT_AddColumn:       /* 添加列 */case AT_AddColumnToView: /* 通过 CREATE OR REPLACE VIEW 添加列 */（后续代码省略）default: /* 异常情况处理 */ereport(ERROR,(errcode(ERRCODE_UNRECOGNIZED_NODE_TYPE),errmsg("unrecognized alter table type: %d", (int)cmd->subtype))); // 报告无法识别的表更改类型错误break;}// 记录修改时间，这是为了保持对象的元数据是最新的PgObjectType objectType = GetPgObjectTypePgClass(tab->relkind);if (objectType != OBJECT_TYPE_INVALID) {UpdatePgObjectMtime(tab->relid, objectType);}/** 报告子命令给感兴趣的事件触发器。*/EventTriggerCollectAlterTableSubcmd((Node *) cmd, address);// 处理分区DDL命令中涉及的独占锁逻辑
#ifndef ENABLE_MULTIPLE_NODESif (PARTITION_DDL_CMD(cmd->subtype)) {AddPartitionDDLInfo(RelationGetRelid(rel));}
#endif/** 为了保证序列中的下一个子命令能看到到目前为止的变更，* 增加命令计数器。*/CommandCounterIncrement();
}

ATExecAddColumn 函数

  ATExecAddColumn 是 OpenGauss 数据库系统中用于处理 ALTER TABLE ADD COLUMN 操作的函数。这个函数的主要作用是将新列添加到现有的表中，同时确保这一变更符合数据库的完整性和约束要求。ATExecAddColumn 的实现考虑到了多种数据库表类型和存储格式，如行存储表、列存储表、外部表等，它涵盖了从权限检查、数据类型验证到索引和默认值处理的多个方面。
功能概述
  以下是 ATExecAddColumn 函数的主要功能和处理步骤：

  1. 权限检查：
  • 在添加列操作开始前，需要对操作的表进行权限检查，确认执行操作的用户具有足够的权限。这通常在函数的上层调用中完成（例如 ATPrepAddColumn）。
  1. 打开和锁定表：
  • 函数首先需要打开目标表，并对其加锁以防止其他操作干扰。这个锁通常是排他锁，确保在添加列的过程中表结构不被其他操作修改。
  1. 处理列存储表的特殊情况：
  • 对于列存储表，特别是 PAX 格式的表，添加列的操作与行存储表有所不同，可能需要特殊处理，如只更新目录而不实际重写数据。
  1. 数据类型和默认值处理：
  • 确定新列的数据类型，包括从类型名称解析 OID、修饰符（typmod）和排序规则（collation）。
  • 检查并处理列的默认值，如果新列定义中包含默认值，需要将这些默认值存储在系统目录中，同时考虑是否触发表的重写以包含新的默认值。
  1. 更新系统目录：
  • 更新 pg_attribute 系统目录表以包含新列的元数据。
  • 如果新列是 OID 类型，则还需更新 pg_class 来标记表现在包含 OID。
  1. 错误处理和特殊类型表的适配：
  • 对于外部表或 MOT 类型的表，添加列的处理可能需要调用特定的外部表处理函数或 MOT 相关的函数。
  • 对于分布式环境或时间序列数据库特有的表，如 Delta 表，需要进行额外的处理以确保表的结构一致性。
  1. 递归处理和继承：
  • 如果表是继承结构的父表，需要递归地将新列添加到所有子表中。这一步骤需要处理继承计数和本地标志，以确保继承逻辑的正确性。
  1. 处理表的注释和额外属性：
  • 为新列添加注释和其他相关属性。
  1. 完成操作和解锁：
  • 在所有必要的更改完成后，需要更新内部缓存和索引，以确保数据库的内部状态与实际数据保持一致。
  • 最后，解锁表并关闭与之相关的系统表。

  函数源码如下所示（路径：src\gausskernel\optimizer\commands\tablecmds.cpp）

// 定义一个函数，用于处理添加列操作，接收多个参数，包括工作队列、表信息、表关系对象、列定义、是否为OID列等
static ObjectAddress ATExecAddColumn(List** wqueue, AlteredTableInfo* tab, Relation rel, ColumnDef* colDef, bool isOid,bool recurse, bool recursing, bool is_first, char *after_name, LOCKMODE lockmode) {// 获取表的 OIDOid myrelid = RelationGetRelid(rel);// 定义关系变量Relation pgclass = NULL;Relation attrdesc = NULL;Relation cedesc = NULL;// 堆元组变量初始化HeapTuple reltup = NULL;// 定义一个新的 pg_attribute 表单数据结构FormData_pg_attribute attribute;// 新列号和当前列号变量初始化int newattnum = 0;int currattnum = 0;// 表类型char relkind;// 类型元组和类型 OIDHeapTuple typeTuple;Oid typeOid = InvalidOid;// 类型修饰符和排序规则 OIDint32 typmod = -1;Oid collOid = InvalidOid;// 类型表单和默认值表达式Form_pg_type tform = NULL;Expr* defval = NULL;// 子表列表和遍历用的 ListCellList* children = NIL;ListCell* child = NULL;// 访问控制结果AclResult aclresult;// 检测是否为列存储表bool isDfsTable = RelationIsPAXFormat(rel);// 对象地址初始化ObjectAddress address;// 判断是否为首列添加或指定位置添加bool is_addloc = is_first || after_name != NULL;// 查询字符串列表初始化，用于执行动态 SQLList *query_str = NIL;// 若为递归调用，则进行简单的权限检查if (recursing)ATSimplePermissions(rel, ATT_TABLE);// 打开 pg_attribute 表attrdesc = heap_open(AttributeRelationId, RowExclusiveLock);// 对于首列添加或指定位置添加，进行特定的错误处理if (is_addloc) {if (rel->rd_rel->relkind == RELKIND_FOREIGN_TABLE) {ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),errmsg("Unsupported feature"),errdetail("foreign table is not supported for add column first|after columnName")));}if (RelationIsColumnFormat(rel)) {ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),errmsg("Unsupported feature"),errdetail("column orientated table is not supported for add column first|after columnName")));}}// 如果添加的是加密列CeHeapInfo* ceHeapInfo = NULL;if (colDef->clientLogicColumnRef != NULL) {if (is_addloc) {ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),errmsg("Unsupported feature"),errdetail("encryption column is not supported for add column first|after columnName")));}if (colDef->clientLogicColumnRef != NULL) {ceHeapInfo = (CeHeapInfo *)palloc(sizeof(CeHeapInfo));process_encrypted_columns(colDef, ceHeapInfo);}cedesc = heap_open(ClientLogicCachedColumnsId, RowExclusiveLock);}// 检查是否在递归子表中添加列，若是，则检查是否需要与现有定义合并if (colDef->inhcount > 0) {HeapTuple tuple;// 在子表中查找是否已存在同名列tuple = SearchSysCacheCopyAttName(myrelid, colDef->colname);if (HeapTupleIsValid(tuple)) {Form_pg_attribute childatt = (Form_pg_attribute)GETSTRUCT(tuple);// 子列必须在类型、类型修饰符和排序规则上与新列匹配Oid ctypeId = InvalidOid;int32 ctypmod = -1;Oid ccollid = InvalidOid;typenameTypeIdAndMod(NULL, colDef->typname, &ctypeId, &ctypmod);if (ctypeId != childatt->atttypid || ctypmod != childatt->atttypmod)ereport(ERROR,(errcode(ERRCODE_DATATYPE_MISMATCH),errmsg("child table \"%s\" has different type for column \"%s\"",RelationGetRelationName(rel),colDef->colname)));ccollid = GetColumnDefCollation(NULL, colDef, ctypeId);if (ccollid != childatt->attcollation)ereport(ERROR,(errcode(ERRCODE_COLLATION_MISMATCH),errmsg("child table \"%s\" has different collation for column \"%s\"",RelationGetRelationName(rel),colDef->colname),errdetail("\"%s\" versus \"%s\"",get_collation_name(ccollid),get_collation_name(childatt->attcollation))));// 如果是 OID 列，子列也必须是 OIDif (isOid && childatt->attnum != ObjectIdAttributeNumber)ereport(ERROR,(errcode(ERRCODE_DATATYPE_MISMATCH),errmsg("child table \"%s\" has a conflicting \"%s\" column",RelationGetRelationName(rel),colDef->colname)));// 增加现有子列的 inhcountchildatt->attinhcount++;simple_heap_update(attrdesc, &tuple->t_self, tuple);CatalogUpdateIndexes(attrdesc, tuple);tableam_tops_free_tuple(tuple);// 通知用户关于合并的信息ereport(NOTICE,(errmsg("merging definition of column \"%s\" for child \"%s\"",colDef->colname,RelationGetRelationName(rel))));// 关闭 pg_attribute 表并返回heap_close(attrdesc, RowExclusiveLock);return InvalidObjectAddress;}}// 打开 pg_class 表pgclass = heap_open(RelationRelationId, RowExclusiveLock);// 从系统缓存中复制 pg_class 元组reltup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(myrelid));if (!HeapTupleIsValid(reltup)) {ereport(ERROR, (errcode(ERRCODE_CACHE_LOOKUP_FAILED), errmsg("cache lookup failed for relation %u", myrelid)));}relkind = ((Form_pg_class)GETSTRUCT(reltup))->relkind;// 检查新名称是否已存在check_for_column_name_collision(rel, colDef->colname);// 确定新属性的编号if (isOid) {newattnum = ObjectIdAttributeNumber;} else {currattnum = ((Form_pg_class)GETSTRUCT(reltup))->relnatts;if (currattnum + 1 > MaxHeapAttributeNumber) {ereport(ERROR,(errcode(ERRCODE_TOO_MANY_COLUMNS),errmsg("tables can have at most %d columns", MaxHeapAttributeNumber)));}if (is_first) {newattnum = 1;} else if (after_name != NULL) {newattnum = GetAfterColumnAttnum(myrelid, after_name);} else {newattnum = currattnum + 1;}}// 获取类型元组和设置类型 OIDtypeTuple = typenameType(NULL, colDef->typname, &typmod);tform = (Form_pg_type)GETSTRUCT(typeTuple);typeOid = HeapTupleGetOid(typeTuple);// 获取列的排序规则Oid rel_coll_oid = rel->rd_options == NULL ? InvalidOid : ((StdRdOptions*)(rel)->rd_options)->collate;collOid = GetColumnDefCollation(NULL, colDef, typeOid, rel_coll_oid);if (DB_IS_CMPT(B_FORMAT)) {typeOid = binary_need_transform_typeid(typeOid, &collOid);if (RelationIsColStore(rel) || RelationIsTsStore(rel)) {check_unsupported_charset_for_column(collOid, colDef->colname);}}// 检查类型使用权限aclresult = pg_type_aclcheck(typeOid, GetUserId(), ACL_USAGE);if (aclresult != ACLCHECK_OK)aclcheck_error_type(aclresult, typeOid);// 确保数据类型适用于列CheckAttributeType(colDef->colname, typeOid, collOid, list_make1_oid(rel->rd_rel->reltype), false);#ifdef ENABLE_MOTif (relkind == RELKIND_FOREIGN_TABLE && isMOTFromTblOid(RelationGetRelid(rel))) {DetermineColumnCollationForMOTTable(&collOid);}#endif// 检查是否为 Delta 表bool isDelta = RELATION_IS_DELTA(rel);// 构造新属性的 pg_attribute 条目attribute.attrelid = myrelid;(void)namestrcpy(&(attribute.attname), colDef->colname);attribute.atttypid = typeOid;attribute.attstattarget = (newattnum > 0) ? -1 : 0;attribute.attlen = tform->typlen;attribute.attcacheoff = -1;attribute.atttypmod = typmod;attribute.attnum = newattnum;attribute.attbyval = tform->typbyval;attribute.attndims = list_length(colDef->typname->arrayBounds);attribute.attstorage = tform->typstorage;attribute.attalign = tform->typalign;attribute.attnotnull = colDef->is_not_null;attribute.atthasdef = false;attribute.attisdropped = false;attribute.attislocal = colDef->is_local;attribute.attkvtype = colDef->kvtype;if (!isDelta) {VerifyAttrCompressMode(colDef->cmprs_mode, attribute.attlen, colDef->colname);attribute.attcmprmode = colDef->cmprs_mode;} else {attribute.attcmprmode = ATT_CMPR_NOCOMPRESS;}attribute.attinhcount = colDef->inhcount;attribute.attcollation = collOid;// attribute.attacl 是通过 InsertPgAttributeTuple 处理的ReleaseSysCache(typeTuple);// 如果是行存储表且不支持压缩if (!isDelta && RelationIsRowFormat(rel) &&  ATT_CMPR_NOCOMPRESS < colDef->cmprs_mode&& colDef->cmprs_mode <= ATT_CMPR_NUMSTR) {ereport(ERROR,(errcode(ERRCODE_INVALID_TABLE_DEFINITION),errmsg("row-oriented table does not support compression")));}// 如果是首列添加或指定位置添加，更新 pg_attribute 和其他相关表if (is_addloc) {UpdatePgAttributeFirstAfter(attrdesc, myrelid, newattnum, currattnum, true);UpdatePgDescriptionFirstAfter(rel, newattnum, currattnum, true);UpdatePgIndexFirstAfter(rel, newattnum, currattnum, true);UpdatePgConstraintFirstAfter(rel, newattnum, currattnum, true);UpdatePgConstraintConfkeyFirstAfter(rel, newattnum, currattnum, true);UpdatePgAttrdefFirstAfter(rel, newattnum, currattnum, true);UpdatePgPartitionFirstAfter(rel, newattnum, currattnum, true, false, NULL);UpdatePgTriggerFirstAfter(rel, newattnum, currattnum, true);UpdatePgRlspolicyFirstAfter(rel, newattnum, currattnum, true);query_str = CheckPgRewriteFirstAfter(rel);tab->rewrite |= AT_REWRITE_ALTER_PERSISTENCE;tab->is_first_after = true;}// 插入新的 pg_attribute 元组InsertPgAttributeTuple(attrdesc, &attribute, NULL);// 关闭 pg_attribute 表heap_close(attrdesc, RowExclusiveLock);// 如果添加的是加密列，处理加密列相关的元组插入if (colDef->clientLogicColumnRef != NULL) {ceHeapInfo->attnum = newattnum;insert_gs_sec_encrypted_column_tuple(ceHeapInfo, cedesc, myrelid,  NULL);heap_close(cedesc, RowExclusiveLock);}// 更新 pg_class 元组if (isOid)((Form_pg_class)GETSTRUCT(reltup))->relhasoids = true;else((Form_pg_class)GETSTRUCT(reltup))->relnatts = currattnum + 1;simple_heap_update(pgclass, &reltup->t_self, reltup);// 保持目录索引更新CatalogUpdateIndexes(pgclass, reltup);// 释放元组tableam_tops_free_tuple(reltup);// 执行新属性创建后的钩子InvokeObjectAccessHook(OAT_POST_CREATE, RelationRelationId, myrelid, newattnum, NULL);// 关闭 pg_class 表heap_close(pgclass, RowExclusiveLock);// 使属性的目录条目可见CommandCounterIncrement();// 如果是首列添加或指定位置添加，更新依赖关系和生成列等if (is_addloc) {UpdatePgDependFirstAfter(rel, newattnum, currattnum, true);UpdateGenerateColFirstAfter(rel, newattnum, currattnum, true);UpdateIndexFirstAfter(rel);// 创建或替换视图ReplaceViewQueryFirstAfter(query_str);}// 如果存在默认值，将其存储在目录中if (colDef->raw_default) {RawColumnDefault* rawEnt = NULL;// 对于外部表的特殊处理if (relkind == RELKIND_FOREIGN_TABLE) {#ifdef ENABLE_MOTif (!isMOTFromTblOid(RelationGetRelid(rel)) && !isPostgresFDWFromTblOid(RelationGetRelid(rel))) {#elseif (!isPostgresFDWFromTblOid(RelationGetRelid(rel))) {#endifereport(ERROR, (errcode(ERRCODE_WRONG_OBJECT_TYPE), errmsg("%s on foreign tables are not supported",colDef->generatedCol ? "generated column" : "default values")));}} else if (relkind ==  RELKIND_STREAM) {ereport(ERROR, (errcode(ERRCODE_WRONG_OBJECT_TYPE), errmsg("%s on streams are not supported",colDef->generatedCol ? "generated column" : "default values")));} else if (RelationIsTsStore(rel)) {ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),errmsg("It's not supported to add column with default value for timeseries tables.")));}// 初始化 RawColumnDefault 结构体并处理默认值和生成列rawEnt = (RawColumnDefault*)palloc(sizeof(RawColumnDefault));rawEnt->attnum = attribute.attnum;rawEnt->raw_default = (Node*)copyObject(colDef->raw_default);rawEnt->update_expr = (Node*)copyObject(colDef->update_default);rawEnt->generatedCol = colDef->generatedCol;// 这个函数原本用于 CREATE TABLE，因此它处理的是默认值列表，但这里只处理一个。(void)AddRelationNewConstraints(rel, list_make1(rawEnt), NIL, false, true);// 使额外的目录更改可见CommandCounterIncrement();}// 如果有默认值，告知第三阶段填充默认表达式#ifdef ENABLE_MOTif ((relkind == RELKIND_FOREIGN_TABLE && isMOTFromTblOid(RelationGetRelid(rel)) && attribute.attnum > 0) ||(relkind != RELKIND_VIEW && relkind != RELKIND_COMPOSITE_TYPE && relkind != RELKIND_FOREIGN_TABLE &&relkind != RELKIND_STREAM && relkind != RELKIND_CONTQUERY && attribute.attnum > 0)) {#elseif (relkind != RELKIND_VIEW && relkind != RELKIND_COMPOSITE_TYPE && relkind != RELKIND_FOREIGN_TABLE &&relkind != RELKIND_STREAM && relkind != RELKIND_CONTQUERY && attribute.attnum > 0) {#endif// 测试新列是否非空bool testNotNull = colDef->is_not_null;// 在原地或在线升级期间，不应清除 nailed-in 系统目录的 relcache，这是考虑到可靠性// 此外，目前不支持在原地或在线升级期间为 pg_class、pg_attribute、pg_proc 中的新列添加非 NULL 默认值。if (u_sess->attr.attr_common.IsInplaceUpgrade &&(rel->rd_id == RelationRelationId || rel->rd_id == AttributeRelationId))defval = NULL;elsedefval = (Expr*)build_column_default(rel, attribute.attnum);// 如果默认值为 NULL 并且类型是域类型，可能需要显式处理 NULL 默认值if (defval == NULL && (GetDomainConstraints(typeOid) != NIL || is_addloc)) {Oid baseTypeId;int32 baseTypeMod;Oid baseTypeColl;baseTypeMod = typmod;baseTypeId = getBaseTypeAndTypmod(typeOid, &baseTypeMod);baseTypeColl = get_typcollation(baseTypeId);defval = (Expr*)makeNullConst(baseTypeId, baseTypeMod, baseTypeColl);if (GetDomainConstraints(typeOid) != NIL) {defval = (Expr*)coerce_to_target_type(NULL, (Node*)defval, baseTypeId, typeOid, typmod, COERCION_ASSIGNMENT, COERCE_IMPLICIT_CAST, -1);if (defval == NULL) /* 应该不会发生 */ereport(ERROR,(errcode(ERRCODE_UNEXPECTED_NULL_VALUE), errmsg("failed to coerce base type to domain")));}}// 如果 defval 不为 NULL，则根据具体情况处理if (defval != NULL) {// 如果当前是行存储表且必须重写，// 则不适用 alter-table-instantly 特性，同样排除临时表和列表。if (attribute.attnum == RelAutoIncAttrNum(rel)) {if (colDef->is_not_null) {ATExecAppendDefValExpr(attribute.attnum, defval, tab, colDef, true, is_addloc);}} else if (contain_specified_function((Node*)defval, NEXTVALFUNCOID)) {// 不支持默认值为 nextval 表达式的 alter table add columnereport(ERROR,(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),errmsg("It's not supported to alter table add column default with nextval expression.")));} else if (RelationIsCUFormat(rel)) {ATExecAppendDefValExpr(attribute.attnum, defval, tab, colDef, false, false);} else if (tab->rewrite>0 || colDef->generatedCol ||RelationUsesSpaceType(rel->rd_rel->relpersistence) == SP_TEMP) {ATExecAppendDefValExpr(attribute.attnum, defval, tab, colDef, false, true);} else {bytea* value = NULL;AT_INSTANT_DEFAULT_VALUE ret =shouldUpdateAllTuples(defval, attribute.atttypid, attribute.attlen, attribute.attbyval, &value);// 如果默认值为常量且不为空if (ret == DEFAULT_NOT_NULL_CONST) {Assert(value != NULL);updateInitDefVal(value, rel, attribute.attnum);pfree_ext(value);// 新列有常数默认值，// 因此不需要进行非空检测。testNotNull = false;} else if (ret == DEFAULT_OTHER) {if (isDfsTable) {ereport(ERROR,(errcode(ERRCODE_INVALID_TABLE_DEFINITION),(errmsg("It is not supported on DFS table. The detailed reasons are the"" followings:"),errdetail("1. the default value may be a volatile function.\n""2. the storage length of default value may be greater than 127.\n""3. the data type of new column is not supported."))));}ATExecAppendDefValExpr(attribute.attnum, defval, tab, colDef, false, false);}// 如果默认值为 NULL，无需进一步操作/* nothing to do if ret is DEFAULT_NULL */}}// 如果新列为 NOT NULL，告诉第三阶段需要检查这一点。// 注意：我们不对 OID 列这样做。OID 会被标记为 not null，但由于它特别填充，无需检查。if (testNotNull) {tab->new_notnull = true;}}// 如果我们正在添加一个 OID 列，我们必须告诉第三阶段重写表以修复这一点。if (isOid) {tab->rewrite |= AT_REWRITE_ALTER_OID;}// 如果表为列存储，处理相关逻辑if (RelationIsColStore(rel)) {// DFS表不重写数据，只更新目录中的默认值if (isDfsTable) {tab->rewrite = AT_REWRITE_ALTER_OID;} else {tab->rewrite |= AT_REWRITE_ALTER_OID;}}// 为新列添加数据类型和排序规则的依赖关系add_column_datatype_dependency(myrelid, newattnum, attribute.atttypid);add_column_collation_dependency(myrelid, newattnum, attribute.attcollation);#ifdef ENABLE_MOT// 如果是外部表并且表是MOT类型，执行MOT特有的ALTER TABLE处理if (relkind == RELKIND_FOREIGN_TABLE && isMOTFromTblOid(RelationGetRelid(rel))) {AlterForeingTableCmd fcmd = {T_AlterForeingTableCmd,AT_AddColumn,rel,nullptr,(Node*)colDef,typeOid,defval};ATExecMOTAlterTable(&fcmd);}#endif#ifdef ENABLE_MULTIPLE_NODES// 如果表是时间序列存储表if (unlikely(RelationIsTsStore(rel))) {// 如果新增的列是标签类型if (colDef->kvtype == ATT_KV_TAG) {// 获取标签表的 OIDOid tag_relid = get_tag_relid(RelationGetRelationName(rel), rel->rd_rel->relnamespace);Relation tagrel;AlteredTableInfo* tagtab = NULL;List* index_col_name = NIL;// 打开标签表tagrel = heap_open(tag_relid, lockmode);// 检查标签表是否正在被使用CheckTableNotInUse(tagrel, "ALTER TABLE");// 获取或创建一个修改信息结构用于记录对标签表的修改tagtab = ATGetQueueEntry(wqueue, tagrel);// 递归调用 ATExecAddColumn 来处理标签表，添加新列ATExecAddColumn(wqueue, tagtab, tagrel, colDef, isOid, false, false, false, NULL, lockmode);char tag_relname[NAMEDATALEN] = {0};// 生成标签表的元数据表名称Tsdb::GenMetaRelname(rel->rd_rel->relnamespace, Tsdb::MetaTableType::META_TABLE_TAGS,tag_relname, TsConf::MAX_TS_NAME_LEN, RelationGetRelationName(rel));// 为新标签列创建索引index_col_name = lappend(index_col_name, colDef->colname);create_tag_index(tag_relid, tag_relname, index_col_name);// 释放列表资源list_free_ext(index_col_name);// 关闭标签表heap_close(tagrel, NoLock);} else if (colDef->kvtype == ATT_KV_FIELD && Tsdb::RelationEnablesTsdbDelta(rel)) {// 如果新增的列是字段类型并且表启用了时间序列Delta存储// 获取Delta表关系Relation delta_rel = Tsdb::RelationGetDeltaRelation(rel, lockmode);// 检查Delta表是否正在被使用CheckTableNotInUse(delta_rel, "ALTER TABLE");// 获取或创建一个修改信息结构用于记录对Delta表的修改AlteredTableInfo* delta_tab = ATGetQueueEntry(wqueue, delta_rel);// 在Delta表上执行添加列操作ATExecAddColumn(wqueue, delta_tab, delta_rel, colDef, isOid, false, false, false, NULL, lockmode);// 关闭Delta表heap_close(delta_rel, NoLock);}}#endif   /* ENABLE_MULTIPLE_NODES */// 如果表是PAX格式，即列存储表if (RelationIsPAXFormat(rel)) {// 为delta表添加列children = lappend_oid(children, RelationGetDeltaRelId(rel));// 获取锁以同步防止并发删除操作LockRelationOid(RelationGetDeltaRelId(rel), lockmode);elog(DEBUG1,"[GET LOCK] Successfully acquired the lock %d on the delta table of %s for altering operation.",lockmode,RelationGetRelationName(rel));#ifdef ENABLE_MULTIPLE_NODES} else if (g_instance.attr.attr_storage.enable_delta_store && RelationIsCUFormat(rel)) {#else// 在集中模式下，delta表可能有唯一索引。当检查唯一约束时，将使用delta表上的唯一索引。因此，这里忽略enable_delta_store标志并同时修改delta表。} else if (RelationIsCUFormat(rel)) {#endif// 添加C-Store关系的delta表到递归处理中，如果列支持继承特性// 我们还需要调用find_inheritance_childrenchildren = find_cstore_delta(rel, lockmode);} else {// 适当地将操作传播到子表。与大多数其他ALTER操作不同，我们必须逐级递归处理，不能一次性使用find_all_inheritors完成。children = find_inheritance_children(RelationGetRelid(rel), lockmode);}// 如果我们被告知不要递归，而且确实有子表存在，那么添加操作会使它们不同步。if (children && !recurse)ereport(ERROR, (errcode(ERRCODE_INVALID_TABLE_DEFINITION), errmsg("column must be added to child tables too")));// 子表应视列为单一继承，Cstore表和delta表不是继承表if (!recursing && !RelationIsCUFormat(rel)) {colDef = (ColumnDef*)copyObject(colDef);colDef->inhcount = 1;colDef->is_local = false;}// 添加列的注释ATCreateColumComments(myrelid, colDef);// 遍历所有子表foreach (child, children) {Oid childrelid = lfirst_oid(child);Relation childrel;AlteredTableInfo* childtab = NULL;// find_inheritance_children已经获取了锁childrel = heap_open(childrelid, NoLock);CheckTableNotInUse(childrel, "ALTER TABLE");// 查找或创建子表的工作队列条目childtab = ATGetQueueEntry(wqueue, childrel);// 递归到子表ATExecAddColumn(wqueue, childtab, childrel, colDef, isOid, recurse, true, is_first, after_name, lockmode);heap_close(childrel, NoLock);}ObjectAddressSubSet(address, RelationRelationId, myrelid, newattnum);return address;
}

ATRewriteTables 函数

  ATRewriteTables 是 ALTER TABLE 操作的第三阶段，处理表结构重写，如改变列的数据类型或添加/删除 OID 等。它会根据表的种类（如普通表、分区表、索引等）和存储格式（如行存、列存）来选择适当的处理函数。针对系统表、临时表等有特殊要求的表进行了错误处理。同时，它还处理触发器事件和数据依赖关系，确保数据库的一致性和完整性。函数源码如下所示（路径：src\gausskernel\optimizer\commands\tablecmds.cpp）

/** ATRewriteTables: ALTER TABLE phase 3*/
static void ATRewriteTables(AlterTableStmt *parsetree, List** wqueue, LOCKMODE lockmode)
{ListCell* ltab = NULL;/* 遍历每个需要检查或重写的表 */foreach (ltab, *wqueue) {AlteredTableInfo* tab = (AlteredTableInfo*)lfirst(ltab);  // 获取表的修改信息对象int rel_format_idx = IDX_ROW_TBL;  // 默认索引为行存储表int idxPartitionedOrNot = IDX_ORDINARY_TBL;  // 默认索引为非分区表#ifdef PGXC/* 检查是否存在与数据重新分配不兼容的表重写操作 */if (tab->rewrite > 0 && list_length(tab->subcmds[AT_PASS_DISTRIB]) > 0 && IS_PGXC_COORDINATOR && !IsConnFromCoord())ereport(ERROR,(errcode(ERRCODE_STATEMENT_TOO_COMPLEX), errmsg("Incompatible operation with data redistribution")));
#endif/* 如果是外部表或流表，则跳过，因为它们没有存储 */if (tab->relkind == RELKIND_FOREIGN_TABLE || tab->relkind == RELKIND_STREAM)continue;/* 根据表的类型打开表并设置格式和分区类型的索引 */if (tab->relkind == RELKIND_RELATION) {Relation temprel = heap_open(tab->relid, NoLock);rel_format_idx =RelationIsCUFormat(temprel) ? IDX_COL_TBL : IDX_ROW_TBL;idxPartitionedOrNot = RELATION_IS_PARTITIONED(temprel) ? IDX_PARTITIONED_TBL : IDX_ORDINARY_TBL;heap_close(temprel, NoLock);} else if (tab->relkind == RELKIND_INDEX || tab->relkind == RELKIND_GLOBAL_INDEX) {Relation temprel = index_open(tab->relid, NoLock);rel_format_idx = IDX_ROW_TBL; // 索引默认为行关系idxPartitionedOrNot = RelationIsPartitioned(temprel) ? IDX_PARTITIONED_TBL : IDX_ORDINARY_TBL;index_close(temprel, NoLock);}/* 检查是否需要因类型变更或默认值添加而传播更改 */if (tab->newvals != NIL || tab->rewrite > 0) {Relation rel;rel = heap_open(tab->relid, NoLock);find_composite_type_dependencies(rel->rd_rel->reltype, rel, NULL);heap_close(rel, NoLock);}/* 如果需要重写表，则构建临时表并复制数据 */if (tab->rewrite > 0) {Relation OldHeap;Oid NewTableSpace;OldHeap = heap_open(tab->relid, NoLock);if (parsetree)EventTriggerTableRewrite((Node *)parsetree, tab->relid, tab->rewrite);if (IsSystemRelation(OldHeap))ereport(ERROR,(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),errmsg("cannot rewrite system relation \"%s\"", RelationGetRelationName(OldHeap))));if (RelationIsUsedAsCatalogTable(OldHeap))ereport(ERROR,(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),errmsg("cannot rewrite table \"%s\" used as a catalog table", RelationGetRelationName(OldHeap))));if (RELATION_IS_OTHER_TEMP(OldHeap))ereport(ERROR,(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),errmsg("cannot rewrite temporary tables of other sessions")));if (tab->newTableSpace)NewTableSpace = tab->newTableSpace;elseNewTableSpace = OldHeap->rd_rel->reltablespace;heap_close(OldHeap, NoLock);ExecRewriteFuncPtrArray[rel_format_idx][idxPartitionedOrNot](tab, NewTableSpace, lockmode);} else {/* 如果没有重写需求，仅测试新约束或不变更表空间时的操作 */if (tab->constraints != NIL || tab->new_notnull) {ExecOnlyTestFuncPtrArray[rel_format_idx][idxPartitionedOrNot](tab);}if (tab->newTableSpace) {CheckTopRelationIsInMyTempSession(tab->relid);if (IS_PGXC_COORDINATOR && !IsConnFromCoord() && IsTransactionBlock() && rel_format_idx == 0 &&tab->relkind == RELKIND_RELATION) {ereport(ERROR,(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),errmsg("alter row table tablespace cannot run inside a transaction block")));}ExecChangeTabspcFuncPtrArray[rel_format_idx][idxPartitionedOrNot](tab, lockmode);}}}
}

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