{"Entry":{"collection":"storage","key":"tuple-layout","name":"Table row layout","aliases":[],"metadata":{"aliases":[],"category":"Physical structures","content_hash":"d5a72f482bf43cb255bd9db27433f977d58f113d28d0d3a7d6a571464b14dda2","imported_at":"2026-09-30T00:40:47.109034+08:00","name":"Table row layout","name_zh":"","slug":"tuple-layout","summary":"Physical row headers, null bitmap, alignment and column data."}},"Definition":{"Collection":"storage","Key":"tuple-layout","SourceDatabase":"center","Version":"18","SourceTable":"storage_structure","SourceKey":"tuple-layout","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","Facts":{"comparison_data":{"layouts":[{"columns":[{"key":"c0","label":"Field"},{"key":"c1","label":"Type"},{"key":"c2","label":"Length"},{"key":"c3","label":"Description"}],"key":"layout-2","rows":[{"c0":"t_xmin","c1":"TransactionId","c2":"4 bytes","c3":"insert XID stamp"},{"c0":"t_xmax","c1":"TransactionId","c2":"4 bytes","c3":"delete XID stamp"},{"c0":"t_cid","c1":"CommandId","c2":"4 bytes","c3":"insert and/or delete CID stamp (overlays with t_xvac)"},{"c0":"t_xvac","c1":"TransactionId","c2":"4 bytes","c3":"XID for VACUUM operation moving a row version"},{"c0":"t_ctid","c1":"ItemPointerData","c2":"6 bytes","c3":"current TID of this or newer row version"},{"c0":"t_infomask2","c1":"uint16","c2":"2 bytes","c3":"number of attributes, plus various flag bits"},{"c0":"t_infomask","c1":"uint16","c2":"2 bytes","c3":"various flag bits"},{"c0":"t_hoff","c1":"uint8","c2":"1 byte","c3":"offset to user data"}],"title":"HeapTupleHeaderData Layout"}]},"comparison_hash":"7f6e968716966dae8869d6c14f0a6aa229d745d4d4bde899b61eeec58c8d7ea0","description":["Physical row headers, null bitmap, alignment and column data."],"facts":[],"manual_html":"\u003cdiv class=\"sect2\"\u003e\n\u003cdiv class=\"titlepage\"\u003e\n\u003cdiv\u003e\n\u003cdiv\u003e\n\u003ch3 class=\"title\"\u003e66.6.1. Table Row Layout \u003c/h3\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eAll table rows are structured in the same way. There is a fixed-size header (occupying 23 bytes on most machines), followed by an optional null bitmap, an optional object ID field, and the user data. The header is detailed in \u003ca class=\"xref\" href=\"/docs/18/storage-page-layout.html#HEAPTUPLEHEADERDATA-TABLE\" title=\"Table 66.4. HeapTupleHeaderData Layout\"\u003eTable 66.4\u003c/a\u003e. The actual user data (columns of the row) begins at the offset indicated by \u003ccode class=\"structfield\"\u003et_hoff\u003c/code\u003e, which must always be a multiple of the MAXALIGN distance for the platform. The null bitmap is only present if the \u003cem class=\"firstterm\"\u003eHEAP_HASNULL\u003c/em\u003e bit is set in \u003ccode class=\"structfield\"\u003et_infomask\u003c/code\u003e. If it is present it begins just after the fixed header and occupies enough bytes to have one bit per data column (that is, the number of bits that equals the attribute count in \u003ccode class=\"structfield\"\u003et_infomask2\u003c/code\u003e). In this list of bits, a 1 bit indicates not-null, a 0 bit is a null. When the bitmap is not present, all columns are assumed not-null. The object ID is only present if the \u003cem class=\"firstterm\"\u003eHEAP_HASOID_OLD\u003c/em\u003e bit is set in \u003ccode class=\"structfield\"\u003et_infomask\u003c/code\u003e. If present, it appears just before the \u003ccode class=\"structfield\"\u003et_hoff\u003c/code\u003e boundary. Any padding needed to make \u003ccode class=\"structfield\"\u003et_hoff\u003c/code\u003e a MAXALIGN multiple will appear between the null bitmap and the object ID. (This in turn ensures that the object ID is suitably aligned.)\u003c/p\u003e\n\u003cdiv class=\"table\"\u003e\n\u003cp class=\"title\"\u003e\u003cstrong\u003eTable 66.4. HeapTupleHeaderData Layout\u003c/strong\u003e\u003c/p\u003e\n\u003cdiv class=\"table-contents\"\u003e\n\u003ctable class=\"table\"\u003e\n\n\n\n\n\n\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth\u003eField\u003c/th\u003e\n\u003cth\u003eType\u003c/th\u003e\n\u003cth\u003eLength\u003c/th\u003e\n\u003cth\u003eDescription\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd\u003et_xmin\u003c/td\u003e\n\u003ctd\u003eTransactionId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003einsert XID stamp\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_xmax\u003c/td\u003e\n\u003ctd\u003eTransactionId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003edelete XID stamp\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_cid\u003c/td\u003e\n\u003ctd\u003eCommandId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003einsert and/or delete CID stamp (overlays with t_xvac)\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_xvac\u003c/td\u003e\n\u003ctd\u003eTransactionId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003eXID for VACUUM operation moving a row version\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_ctid\u003c/td\u003e\n\u003ctd\u003eItemPointerData\u003c/td\u003e\n\u003ctd\u003e6 bytes\u003c/td\u003e\n\u003ctd\u003ecurrent TID of this or newer row version\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_infomask2\u003c/td\u003e\n\u003ctd\u003euint16\u003c/td\u003e\n\u003ctd\u003e2 bytes\u003c/td\u003e\n\u003ctd\u003enumber of attributes, plus various flag bits\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_infomask\u003c/td\u003e\n\u003ctd\u003euint16\u003c/td\u003e\n\u003ctd\u003e2 bytes\u003c/td\u003e\n\u003ctd\u003evarious flag bits\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_hoff\u003c/td\u003e\n\u003ctd\u003euint8\u003c/td\u003e\n\u003ctd\u003e1 byte\u003c/td\u003e\n\u003ctd\u003eoffset to user data\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003c/div\u003e\u003cbr class=\"table-break\"\u003e\n\u003cp\u003eAll the details can be found in \u003ccode class=\"filename\"\u003esrc/include/access/htup_details.h\u003c/code\u003e.\u003c/p\u003e\n\u003cp\u003eInterpreting the actual data can only be done with information obtained from other tables, mostly \u003ccode class=\"structname\"\u003epg_attribute\u003c/code\u003e. The key values needed to identify field locations are \u003ccode class=\"structfield\"\u003eattlen\u003c/code\u003e and \u003ccode class=\"structfield\"\u003eattalign\u003c/code\u003e. There is no way to directly get a particular attribute, except when there are only fixed width fields and no null values. All this trickery is wrapped up in the functions \u003cem class=\"firstterm\"\u003eheap_getattr\u003c/em\u003e, \u003cem class=\"firstterm\"\u003efastgetattr\u003c/em\u003e and \u003cem class=\"firstterm\"\u003eheap_getsysattr\u003c/em\u003e.\u003c/p\u003e\n\u003cp\u003eTo read the data you need to examine each attribute in turn. First check whether the field is NULL according to the null bitmap. If it is, go to the next. Then make sure you have the right alignment. If the field is a fixed width field, then all the bytes are simply placed. If it's a variable length field (attlen = -1) then it's a bit more complicated. All variable-length data types share the common header structure \u003ccode class=\"type\"\u003estruct varlena\u003c/code\u003e, which includes the total length of the stored value and some flag bits. Depending on the flags, the data can be either inline or in a TOAST table; it might be compressed, too (see \u003ca class=\"xref\" href=\"/docs/18/storage-toast.html\" title=\"66.2. TOAST\"\u003eSection 66.2\u003c/a\u003e).\u003c/p\u003e\n\u003c/div\u003e","manual_path":"/docs/18/storage-page-layout.html","related":[],"release":{"channel":"stable","label":"18.6","major":"18","ref":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2","revision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f"},"sections":[],"signature":"","sources":[{"label":"PostgreSQL 18 English manual","path":"storage-page-layout.html","sha256":"0d9f99e4cef5ea0a6741a74cab87052b929f7001730853c85d5e60d72179d355","url":"/docs/18/storage-page-layout.html#STORAGE-TUPLE-LAYOUT"}],"tables":[{"columns":[{"key":"c0","label":"Field"},{"key":"c1","label":"Type"},{"key":"c2","label":"Length"},{"key":"c3","label":"Description"}],"key":"layout-2","rows":[{"c0":"t_xmin","c1":"TransactionId","c2":"4 bytes","c3":"insert XID stamp"},{"c0":"t_xmax","c1":"TransactionId","c2":"4 bytes","c3":"delete XID stamp"},{"c0":"t_cid","c1":"CommandId","c2":"4 bytes","c3":"insert and/or delete CID stamp (overlays with t_xvac)"},{"c0":"t_xvac","c1":"TransactionId","c2":"4 bytes","c3":"XID for VACUUM operation moving a row version"},{"c0":"t_ctid","c1":"ItemPointerData","c2":"6 bytes","c3":"current TID of this or newer row version"},{"c0":"t_infomask2","c1":"uint16","c2":"2 bytes","c3":"number of attributes, plus various flag bits"},{"c0":"t_infomask","c1":"uint16","c2":"2 bytes","c3":"various flag bits"},{"c0":"t_hoff","c1":"uint8","c2":"1 byte","c3":"offset to user data"}],"title":"HeapTupleHeaderData Layout"}]},"ManualEvidence":{"manual_path":"/docs/18/storage-page-layout.html","release":{"channel":"stable","label":"18.6","major":"18","ref":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2","revision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f"},"sources":[{"label":"PostgreSQL 18 English manual","path":"storage-page-layout.html","sha256":"0d9f99e4cef5ea0a6741a74cab87052b929f7001730853c85d5e60d72179d355","url":"/docs/18/storage-page-layout.html#STORAGE-TUPLE-LAYOUT"}]},"MeasuredEvidence":{}},"Text":{"Collection":"storage","Key":"tuple-layout","SourceDatabase":"center","Version":"18","Locale":"en","Title":"Table row layout","Summary":"Physical row headers, null bitmap, alignment and column data.","BodyHTML":"\u003cdiv\u003e\n\u003cdiv\u003e\n\u003cdiv\u003e\n\u003cdiv\u003e\n\u003ch3\u003e66.6.1. Table Row Layout \u003c/h3\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eAll table rows are structured in the same way. There is a fixed-size header (occupying 23 bytes on most machines), followed by an optional null bitmap, an optional object ID field, and the user data. The header is detailed in \u003ca href=\"/docs/18/storage-page-layout.html#HEAPTUPLEHEADERDATA-TABLE\" rel=\"nofollow\"\u003eTable 66.4\u003c/a\u003e. The actual user data (columns of the row) begins at the offset indicated by \u003ccode\u003et_hoff\u003c/code\u003e, which must always be a multiple of the MAXALIGN distance for the platform. The null bitmap is only present if the \u003cem\u003eHEAP_HASNULL\u003c/em\u003e bit is set in \u003ccode\u003et_infomask\u003c/code\u003e. If it is present it begins just after the fixed header and occupies enough bytes to have one bit per data column (that is, the number of bits that equals the attribute count in \u003ccode\u003et_infomask2\u003c/code\u003e). In this list of bits, a 1 bit indicates not-null, a 0 bit is a null. When the bitmap is not present, all columns are assumed not-null. The object ID is only present if the \u003cem\u003eHEAP_HASOID_OLD\u003c/em\u003e bit is set in \u003ccode\u003et_infomask\u003c/code\u003e. If present, it appears just before the \u003ccode\u003et_hoff\u003c/code\u003e boundary. Any padding needed to make \u003ccode\u003et_hoff\u003c/code\u003e a MAXALIGN multiple will appear between the null bitmap and the object ID. (This in turn ensures that the object ID is suitably aligned.)\u003c/p\u003e\n\u003cdiv\u003e\n\u003cp\u003e\u003cstrong\u003eTable 66.4. HeapTupleHeaderData Layout\u003c/strong\u003e\u003c/p\u003e\n\u003cdiv\u003e\n\u003ctable\u003e\n\n\n\n\n\n\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth\u003eField\u003c/th\u003e\n\u003cth\u003eType\u003c/th\u003e\n\u003cth\u003eLength\u003c/th\u003e\n\u003cth\u003eDescription\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd\u003et_xmin\u003c/td\u003e\n\u003ctd\u003eTransactionId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003einsert XID stamp\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_xmax\u003c/td\u003e\n\u003ctd\u003eTransactionId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003edelete XID stamp\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_cid\u003c/td\u003e\n\u003ctd\u003eCommandId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003einsert and/or delete CID stamp (overlays with t_xvac)\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_xvac\u003c/td\u003e\n\u003ctd\u003eTransactionId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003eXID for VACUUM operation moving a row version\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_ctid\u003c/td\u003e\n\u003ctd\u003eItemPointerData\u003c/td\u003e\n\u003ctd\u003e6 bytes\u003c/td\u003e\n\u003ctd\u003ecurrent TID of this or newer row version\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_infomask2\u003c/td\u003e\n\u003ctd\u003euint16\u003c/td\u003e\n\u003ctd\u003e2 bytes\u003c/td\u003e\n\u003ctd\u003enumber of attributes, plus various flag bits\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_infomask\u003c/td\u003e\n\u003ctd\u003euint16\u003c/td\u003e\n\u003ctd\u003e2 bytes\u003c/td\u003e\n\u003ctd\u003evarious flag bits\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_hoff\u003c/td\u003e\n\u003ctd\u003euint8\u003c/td\u003e\n\u003ctd\u003e1 byte\u003c/td\u003e\n\u003ctd\u003eoffset to user data\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003c/div\u003e\u003cbr\u003e\n\u003cp\u003eAll the details can be found in \u003ccode\u003esrc/include/access/htup_details.h\u003c/code\u003e.\u003c/p\u003e\n\u003cp\u003eInterpreting the actual data can only be done with information obtained from other tables, mostly \u003ccode\u003epg_attribute\u003c/code\u003e. The key values needed to identify field locations are \u003ccode\u003eattlen\u003c/code\u003e and \u003ccode\u003eattalign\u003c/code\u003e. There is no way to directly get a particular attribute, except when there are only fixed width fields and no null values. All this trickery is wrapped up in the functions \u003cem\u003eheap_getattr\u003c/em\u003e, \u003cem\u003efastgetattr\u003c/em\u003e and \u003cem\u003eheap_getsysattr\u003c/em\u003e.\u003c/p\u003e\n\u003cp\u003eTo read the data you need to examine each attribute in turn. First check whether the field is NULL according to the null bitmap. If it is, go to the next. Then make sure you have the right alignment. If the field is a fixed width field, then all the bytes are simply placed. If it\u0026#39;s a variable length field (attlen = -1) then it\u0026#39;s a bit more complicated. All variable-length data types share the common header structure \u003ccode\u003estruct varlena\u003c/code\u003e, which includes the total length of the stored value and some flag bits. Depending on the flags, the data can be either inline or in a TOAST table; it might be compressed, too (see \u003ca href=\"/docs/18/storage-toast.html\" rel=\"nofollow\"\u003eSection 66.2\u003c/a\u003e).\u003c/p\u003e\n\u003c/div\u003e","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","ContentHash":"787d08d6acad13fa22e89999147bdc3f14fcc04438cd4ee835834e1bebc9a236","Payload":{"description":["Physical row headers, null bitmap, alignment and column data."],"manual_html":"\u003cdiv class=\"sect2\"\u003e\n\u003cdiv class=\"titlepage\"\u003e\n\u003cdiv\u003e\n\u003cdiv\u003e\n\u003ch3 class=\"title\"\u003e66.6.1. Table Row Layout \u003c/h3\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eAll table rows are structured in the same way. There is a fixed-size header (occupying 23 bytes on most machines), followed by an optional null bitmap, an optional object ID field, and the user data. The header is detailed in \u003ca class=\"xref\" href=\"/docs/18/storage-page-layout.html#HEAPTUPLEHEADERDATA-TABLE\" title=\"Table 66.4. HeapTupleHeaderData Layout\"\u003eTable 66.4\u003c/a\u003e. The actual user data (columns of the row) begins at the offset indicated by \u003ccode class=\"structfield\"\u003et_hoff\u003c/code\u003e, which must always be a multiple of the MAXALIGN distance for the platform. The null bitmap is only present if the \u003cem class=\"firstterm\"\u003eHEAP_HASNULL\u003c/em\u003e bit is set in \u003ccode class=\"structfield\"\u003et_infomask\u003c/code\u003e. If it is present it begins just after the fixed header and occupies enough bytes to have one bit per data column (that is, the number of bits that equals the attribute count in \u003ccode class=\"structfield\"\u003et_infomask2\u003c/code\u003e). In this list of bits, a 1 bit indicates not-null, a 0 bit is a null. When the bitmap is not present, all columns are assumed not-null. The object ID is only present if the \u003cem class=\"firstterm\"\u003eHEAP_HASOID_OLD\u003c/em\u003e bit is set in \u003ccode class=\"structfield\"\u003et_infomask\u003c/code\u003e. If present, it appears just before the \u003ccode class=\"structfield\"\u003et_hoff\u003c/code\u003e boundary. Any padding needed to make \u003ccode class=\"structfield\"\u003et_hoff\u003c/code\u003e a MAXALIGN multiple will appear between the null bitmap and the object ID. (This in turn ensures that the object ID is suitably aligned.)\u003c/p\u003e\n\u003cdiv class=\"table\"\u003e\n\u003cp class=\"title\"\u003e\u003cstrong\u003eTable 66.4. HeapTupleHeaderData Layout\u003c/strong\u003e\u003c/p\u003e\n\u003cdiv class=\"table-contents\"\u003e\n\u003ctable class=\"table\"\u003e\n\n\n\n\n\n\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth\u003eField\u003c/th\u003e\n\u003cth\u003eType\u003c/th\u003e\n\u003cth\u003eLength\u003c/th\u003e\n\u003cth\u003eDescription\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd\u003et_xmin\u003c/td\u003e\n\u003ctd\u003eTransactionId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003einsert XID stamp\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_xmax\u003c/td\u003e\n\u003ctd\u003eTransactionId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003edelete XID stamp\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_cid\u003c/td\u003e\n\u003ctd\u003eCommandId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003einsert and/or delete CID stamp (overlays with t_xvac)\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_xvac\u003c/td\u003e\n\u003ctd\u003eTransactionId\u003c/td\u003e\n\u003ctd\u003e4 bytes\u003c/td\u003e\n\u003ctd\u003eXID for VACUUM operation moving a row version\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_ctid\u003c/td\u003e\n\u003ctd\u003eItemPointerData\u003c/td\u003e\n\u003ctd\u003e6 bytes\u003c/td\u003e\n\u003ctd\u003ecurrent TID of this or newer row version\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_infomask2\u003c/td\u003e\n\u003ctd\u003euint16\u003c/td\u003e\n\u003ctd\u003e2 bytes\u003c/td\u003e\n\u003ctd\u003enumber of attributes, plus various flag bits\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_infomask\u003c/td\u003e\n\u003ctd\u003euint16\u003c/td\u003e\n\u003ctd\u003e2 bytes\u003c/td\u003e\n\u003ctd\u003evarious flag bits\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003et_hoff\u003c/td\u003e\n\u003ctd\u003euint8\u003c/td\u003e\n\u003ctd\u003e1 byte\u003c/td\u003e\n\u003ctd\u003eoffset to user data\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003c/div\u003e\u003cbr class=\"table-break\"\u003e\n\u003cp\u003eAll the details can be found in \u003ccode class=\"filename\"\u003esrc/include/access/htup_details.h\u003c/code\u003e.\u003c/p\u003e\n\u003cp\u003eInterpreting the actual data can only be done with information obtained from other tables, mostly \u003ccode class=\"structname\"\u003epg_attribute\u003c/code\u003e. The key values needed to identify field locations are \u003ccode class=\"structfield\"\u003eattlen\u003c/code\u003e and \u003ccode class=\"structfield\"\u003eattalign\u003c/code\u003e. There is no way to directly get a particular attribute, except when there are only fixed width fields and no null values. All this trickery is wrapped up in the functions \u003cem class=\"firstterm\"\u003eheap_getattr\u003c/em\u003e, \u003cem class=\"firstterm\"\u003efastgetattr\u003c/em\u003e and \u003cem class=\"firstterm\"\u003eheap_getsysattr\u003c/em\u003e.\u003c/p\u003e\n\u003cp\u003eTo read the data you need to examine each attribute in turn. First check whether the field is NULL according to the null bitmap. If it is, go to the next. Then make sure you have the right alignment. If the field is a fixed width field, then all the bytes are simply placed. If it's a variable length field (attlen = -1) then it's a bit more complicated. All variable-length data types share the common header structure \u003ccode class=\"type\"\u003estruct varlena\u003c/code\u003e, which includes the total length of the stored value and some flag bits. Depending on the flags, the data can be either inline or in a TOAST table; it might be compressed, too (see \u003ca class=\"xref\" href=\"/docs/18/storage-toast.html\" title=\"66.2. TOAST\"\u003eSection 66.2\u003c/a\u003e).\u003c/p\u003e\n\u003c/div\u003e","related":[],"sections":[],"tables":[{"columns":[{"key":"c0","label":"Field"},{"key":"c1","label":"Type"},{"key":"c2","label":"Length"},{"key":"c3","label":"Description"}],"key":"layout-2","rows":[{"c0":"t_xmin","c1":"TransactionId","c2":"4 bytes","c3":"insert XID stamp"},{"c0":"t_xmax","c1":"TransactionId","c2":"4 bytes","c3":"delete XID stamp"},{"c0":"t_cid","c1":"CommandId","c2":"4 bytes","c3":"insert and/or delete CID stamp (overlays with t_xvac)"},{"c0":"t_xvac","c1":"TransactionId","c2":"4 bytes","c3":"XID for VACUUM operation moving a row version"},{"c0":"t_ctid","c1":"ItemPointerData","c2":"6 bytes","c3":"current TID of this or newer row version"},{"c0":"t_infomask2","c1":"uint16","c2":"2 bytes","c3":"number of attributes, plus various flag bits"},{"c0":"t_infomask","c1":"uint16","c2":"2 bytes","c3":"various flag bits"},{"c0":"t_hoff","c1":"uint8","c2":"1 byte","c3":"offset to user data"}],"title":"HeapTupleHeaderData Layout"}]}},"RequestedLocale":"zh-Hans","Fallback":true,"Versions":["11","12","13","14","15","16","17","18","19","20"],"Locales":["en"],"Signatures":null,"Spellings":null,"SQLState":null,"Evidence":null}