{"Entry":{"collection":"plan","key":"custom-scan","name":"Custom Scan","aliases":["Custom Scan (%s)","CustomScan","T_CustomScan"],"metadata":{"aliases":["Custom Scan (%s)","CustomScan","T_CustomScan"],"category":"Extensibility","content_hash":"e42bb000e6ab1d49611da52634bee780e2ced0da09b8479c26e1e7fa5ac1ee0a","derived_content_hash":"13d3588bc4b1fe3288144a223aa2ad17a6926a87d7178282393819589b8de936","imported_at":"2026-09-30T00:40:44.1067+08:00","name":"Custom Scan","name_zh":"CustomScan","slug":"custom-scan","summary":"Executes a plan supplied by a custom-scan provider."}},"Definition":{"Collection":"plan","Key":"custom-scan","SourceDatabase":"center","Version":"18","SourceTable":"plan_node","SourceKey":"custom-scan","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","Facts":{"comparison_data":{"initializer":"ExecInitCustomScan","memory_mechanism":"unclassified","node_tag":"T_CustomScan","parallel_callbacks":["ExecCustomScanEstimate","ExecCustomScanInitializeDSM","ExecCustomScanInitializeWorker","ExecCustomScanReInitializeDSM"],"partial_modes":[],"strategies":[],"text_names":["Custom Scan (%s)"]},"comparison_hash":"c905b2ddadbea858aa4811fa7d15ee0733e3c82e2ac2c9fad827846072b5c975","description":["Executes a plan supplied by a custom-scan provider."],"description_zh":"执行自定义扫描提供者生成的计划。","description_zh_html":"\u003cp\u003e执行自定义扫描提供者生成的计划。\u003c/p\u003e\u003cp\u003e核心节点标识：\u003ccode\u003eT_CustomScan\u003c/code\u003e；EXPLAIN 名称：\u003ccode\u003eCustom Scan (%s)\u003c/code\u003e。\u003c/p\u003e\u003cp\u003e本版本源码中的并行执行或观测回调：\u003ccode\u003eExecCustomScanEstimate, ExecCustomScanInitializeDSM, ExecCustomScanInitializeWorker, ExecCustomScanReInitializeDSM\u003c/code\u003e。回调的存在不表示该节点必然支持共享并行扫描；在并行工作进程中执行也不等于具有并行感知能力。\u003c/p\u003e\u003ch3\u003e同版本中文手册节选\u003c/h3\u003e\u003cp\u003e\n\u003cspan class=\"productname\"\u003ePostgreSQL\u003c/span\u003e 会为收到的每个查询制定一个\u003cem class=\"firstterm\"\u003e查询计划\u003c/em\u003e。选择与查询结构和数据特性相匹配的正确计划，对获得良好的性能至关重要，因此系统内置了一个复杂的\u003cem class=\"firstterm\"\u003e规划器\u003c/em\u003e来尽量选出好的计划。可以使用 \u003ca class=\"link\" href=\"/zh/docs/18/sql-explain.html\"\u003e\u003ccode class=\"command\"\u003eEXPLAIN\u003c/code\u003e\u003c/a\u003e 命令查看规划器为任意查询生成的查询计划。读懂计划是一门需要经验积累的技能，本节将尝试介绍其中的基础知识。\n   \u003c/p\u003e\u003cp\u003e\n    本节中的示例取自执行过 \u003ccode class=\"command\"\u003eVACUUM ANALYZE\u003c/code\u003e 的回归测试数据库，使用的是 v18 开发版本源码。如果自行尝试这些示例，通常应当能得到相近的结果，但估计代价和行计数可能略有差异，因为 \u003ccode class=\"command\"\u003eANALYZE\u003c/code\u003e 生成的统计信息来自随机采样而非精确计数，而且代价本身在某种程度上也依赖于平台。\n   \u003c/p\u003e\u003cp\u003e\n    这些示例使用 \u003ccode class=\"command\"\u003eEXPLAIN\u003c/code\u003e 默认的\u003cspan class=\"quote\"\u003e“\u003cspan class=\"quote\"\u003etext\u003c/span\u003e”\u003c/span\u003e输出格式，它紧凑且便于人工阅读。如果希望将 \u003ccode class=\"command\"\u003eEXPLAIN\u003c/code\u003e 的输出交给程序做进一步分析，则应改用机器可读的输出格式（XML、JSON 或 YAML）。\n   \u003c/p\u003e\u003cp\u003e\u003ca href=\"/zh/docs/18/using-explain.html\"\u003e查看完整中文手册与上下文\u003c/a\u003e。\u003c/p\u003e\u003cp\u003e本页开头的中文说明为 PG.CENTER 对编辑说明的翻译；手册节选保留对应版本中文文档原文。源码信息及手册示例不代表重新进行过运行测试。\u003c/p\u003e","evidence_kind":"source and documentation","explain_names":["Custom Scan (%s)"],"explain_prefixes":["Parallel","Async"],"facts":[{"label":"Core node tag","value":"T_CustomScan"},{"label":"Structured EXPLAIN Node Type","value":"Custom Scan"},{"label":"Inputs","value":"Provider-defined plan, possibly with children"},{"label":"Output","value":"Provider-defined tuples"},{"label":"Executor initializer","value":"ExecInitCustomScan"},{"label":"Memory mechanism","value":"unclassified"}],"facts_zh":[{"label":"核心节点标签","value":"T_CustomScan"},{"label":"结构化 EXPLAIN 节点类型","value":"Custom Scan"},{"label":"输入","value":"提供方定义的计划，可能包含子计划"},{"label":"输出","value":"提供方定义的元组"},{"label":"执行器初始化函数","value":"ExecInitCustomScan"},{"label":"内存机制","value":"unclassified"}],"memory":{"description":"This extraction does not assign a universal memory limit or spill policy to this node. 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ExecCustomScanInitializeDSM, ExecCustomScanInitializeWorker, ExecCustomScanReInitializeDSM."],"title":"并行执行与运行信息采集"},{"paragraphs":["nodeCustom.c：处理自定义扫描节点执行的例程。","分配 CustomScanState 对象。由自定义扫描提供者调用 palloc，以便它可以分配一个更大的对象，并将 CustomScanState 嵌入为首字段。此时必须正确设置节点标签和 methods 字段；其他标准字段应置零。","若 CustomScanState 指定了自定义槽位则使用它，否则使用虚拟槽位。","确定扫描元组的类型。如果自定义扫描提供者给出了描述扫描元组的目标列表，则使用该列表；否则使用基表关系的行类型。","自定义扫描提供者的回调按自身逻辑完成自定义扫描状态节点的最后初始化。"],"title":"执行器实现说明"},{"code":"case T_CustomScan:\n\t\t\tsname = \"Custom Scan\";\n\t\t\tcustom_name = ((CustomScan *) plan)-\u003emethods-\u003eCustomName;\n\t\t\tif (custom_name)\n\t\t\t\tpname = psprintf(\"Custom Scan (%s)\", custom_name);\n\t\t\telse\n\t\t\t\tpname = sname;\n\t\t\tbreak;","title":"核心源码中的 EXPLAIN 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