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Move pg_multixact SLRU page format definitions to a separate header
This makes them accessible from pg_upgrade, needed by the next commit. I'm doing this mechanical move as a separate commit to make the next commit's changes to these definitions more obvious. Author: Maxim Orlov <orlovmg@gmail.com> Discussion: https://www.postgresql.org/message-id/CACG%3DezbZo_3_fnx%3DS5BfepwRftzrpJ%2B7WET4EkTU6wnjDTsnjg@mail.gmail.com
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‎src/backend/access/transam/multixact.c‎

Lines changed: 1 addition & 119 deletions
Original file line numberDiff line numberDiff line change
@@ -69,6 +69,7 @@
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#include "postgres.h"
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#include "access/multixact.h"
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#include "access/multixact_internal.h"
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#include "access/slru.h"
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#include "access/twophase.h"
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#include "access/twophase_rmgr.h"
@@ -88,125 +89,6 @@
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#include "utils/memutils.h"
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/*
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* Defines for MultiXactOffset page sizes. A page is the same BLCKSZ as is
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* used everywhere else in Postgres.
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*
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* Note: because MultiXactOffsets are 32 bits and wrap around at 0xFFFFFFFF,
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* MultiXact page numbering also wraps around at
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* 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE, and segment numbering at
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* 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need
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* take no explicit notice of that fact in this module, except when comparing
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* segment and page numbers in TruncateMultiXact (see
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* MultiXactOffsetPagePrecedes).
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*/
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/* We need four bytes per offset */
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#define MULTIXACT_OFFSETS_PER_PAGE (BLCKSZ / sizeof(MultiXactOffset))
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static inline int64
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MultiXactIdToOffsetPage(MultiXactId multi)
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{
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return multi / MULTIXACT_OFFSETS_PER_PAGE;
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}
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static inline int
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MultiXactIdToOffsetEntry(MultiXactId multi)
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{
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return multi % MULTIXACT_OFFSETS_PER_PAGE;
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}
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static inline int64
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MultiXactIdToOffsetSegment(MultiXactId multi)
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{
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return MultiXactIdToOffsetPage(multi) / SLRU_PAGES_PER_SEGMENT;
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}
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/*
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* The situation for members is a bit more complex: we store one byte of
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* additional flag bits for each TransactionId. To do this without getting
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* into alignment issues, we store four bytes of flags, and then the
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* corresponding 4 Xids. Each such 5-word (20-byte) set we call a "group", and
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* are stored as a whole in pages. Thus, with 8kB BLCKSZ, we keep 409 groups
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* per page. This wastes 12 bytes per page, but that's OK -- simplicity (and
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* performance) trumps space efficiency here.
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*
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* Note that the "offset" macros work with byte offset, not array indexes, so
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* arithmetic must be done using "char *" pointers.
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*/
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/* We need eight bits per xact, so one xact fits in a byte */
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#define MXACT_MEMBER_BITS_PER_XACT 8
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#define MXACT_MEMBER_FLAGS_PER_BYTE 1
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#define MXACT_MEMBER_XACT_BITMASK ((1 << MXACT_MEMBER_BITS_PER_XACT) - 1)
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/* how many full bytes of flags are there in a group? */
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#define MULTIXACT_FLAGBYTES_PER_GROUP 4
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#define MULTIXACT_MEMBERS_PER_MEMBERGROUP \
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(MULTIXACT_FLAGBYTES_PER_GROUP * MXACT_MEMBER_FLAGS_PER_BYTE)
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/* size in bytes of a complete group */
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#define MULTIXACT_MEMBERGROUP_SIZE \
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(sizeof(TransactionId) * MULTIXACT_MEMBERS_PER_MEMBERGROUP + MULTIXACT_FLAGBYTES_PER_GROUP)
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#define MULTIXACT_MEMBERGROUPS_PER_PAGE (BLCKSZ / MULTIXACT_MEMBERGROUP_SIZE)
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#define MULTIXACT_MEMBERS_PER_PAGE \
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(MULTIXACT_MEMBERGROUPS_PER_PAGE * MULTIXACT_MEMBERS_PER_MEMBERGROUP)
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/*
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* Because the number of items per page is not a divisor of the last item
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* number (member 0xFFFFFFFF), the last segment does not use the maximum number
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* of pages, and moreover the last used page therein does not use the same
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* number of items as previous pages. (Another way to say it is that the
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* 0xFFFFFFFF member is somewhere in the middle of the last page, so the page
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* has some empty space after that item.)
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*
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* This constant is the number of members in the last page of the last segment.
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*/
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#define MAX_MEMBERS_IN_LAST_MEMBERS_PAGE \
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((uint32) ((0xFFFFFFFF % MULTIXACT_MEMBERS_PER_PAGE) + 1))
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/* page in which a member is to be found */
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static inline int64
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MXOffsetToMemberPage(MultiXactOffset offset)
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{
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return offset / MULTIXACT_MEMBERS_PER_PAGE;
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}
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static inline int64
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MXOffsetToMemberSegment(MultiXactOffset offset)
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{
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return MXOffsetToMemberPage(offset) / SLRU_PAGES_PER_SEGMENT;
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}
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/* Location (byte offset within page) of flag word for a given member */
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static inline int
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MXOffsetToFlagsOffset(MultiXactOffset offset)
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{
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MultiXactOffset group = offset / MULTIXACT_MEMBERS_PER_MEMBERGROUP;
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int grouponpg = group % MULTIXACT_MEMBERGROUPS_PER_PAGE;
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int byteoff = grouponpg * MULTIXACT_MEMBERGROUP_SIZE;
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return byteoff;
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}
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static inline int
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MXOffsetToFlagsBitShift(MultiXactOffset offset)
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{
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int member_in_group = offset % MULTIXACT_MEMBERS_PER_MEMBERGROUP;
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int bshift = member_in_group * MXACT_MEMBER_BITS_PER_XACT;
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return bshift;
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}
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/* Location (byte offset within page) of TransactionId of given member */
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static inline int
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MXOffsetToMemberOffset(MultiXactOffset offset)
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{
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int member_in_group = offset % MULTIXACT_MEMBERS_PER_MEMBERGROUP;
204-
205-
return MXOffsetToFlagsOffset(offset) +
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MULTIXACT_FLAGBYTES_PER_GROUP +
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member_in_group * sizeof(TransactionId);
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}
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/* Multixact members wraparound thresholds. */
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#define MULTIXACT_MEMBER_SAFE_THRESHOLD (MaxMultiXactOffset / 2)
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#define MULTIXACT_MEMBER_DANGER_THRESHOLD \

‎src/include/access/multixact_internal.h‎

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,140 @@
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/*
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* multixact_internal.h
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*
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* PostgreSQL multi-transaction-log manager internal declarations
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*
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* These functions and definitions are for dealing with pg_multixact SLRU
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* pages. They are internal to multixact.c, but they are exported here to
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* allow pg_upgrade to write pg_multixact files directly.
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*
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* Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* src/include/access/multixact_internal.h
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*/
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#ifndef MULTIXACT_INTERNAL_H
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#define MULTIXACT_INTERNAL_H
17+
18+
#include "access/multixact.h"
19+
20+
21+
/*
22+
* Defines for MultiXactOffset page sizes. A page is the same BLCKSZ as is
23+
* used everywhere else in Postgres.
24+
*
25+
* Note: because MultiXactOffsets are 32 bits and wrap around at 0xFFFFFFFF,
26+
* MultiXact page numbering also wraps around at
27+
* 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE, and segment numbering at
28+
* 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need
29+
* take no explicit notice of that fact in this module, except when comparing
30+
* segment and page numbers in TruncateMultiXact (see
31+
* MultiXactOffsetPagePrecedes).
32+
*/
33+
34+
/* We need four bytes per offset */
35+
#define MULTIXACT_OFFSETS_PER_PAGE (BLCKSZ / sizeof(MultiXactOffset))
36+
37+
static inline int64
38+
MultiXactIdToOffsetPage(MultiXactId multi)
39+
{
40+
return multi / MULTIXACT_OFFSETS_PER_PAGE;
41+
}
42+
43+
static inline int
44+
MultiXactIdToOffsetEntry(MultiXactId multi)
45+
{
46+
return multi % MULTIXACT_OFFSETS_PER_PAGE;
47+
}
48+
49+
static inline int64
50+
MultiXactIdToOffsetSegment(MultiXactId multi)
51+
{
52+
return MultiXactIdToOffsetPage(multi) / SLRU_PAGES_PER_SEGMENT;
53+
}
54+
55+
/*
56+
* The situation for members is a bit more complex: we store one byte of
57+
* additional flag bits for each TransactionId. To do this without getting
58+
* into alignment issues, we store four bytes of flags, and then the
59+
* corresponding 4 Xids. Each such 5-word (20-byte) set we call a "group", and
60+
* are stored as a whole in pages. Thus, with 8kB BLCKSZ, we keep 409 groups
61+
* per page. This wastes 12 bytes per page, but that's OK -- simplicity (and
62+
* performance) trumps space efficiency here.
63+
*
64+
* Note that the "offset" macros work with byte offset, not array indexes, so
65+
* arithmetic must be done using "char *" pointers.
66+
*/
67+
/* We need eight bits per xact, so one xact fits in a byte */
68+
#define MXACT_MEMBER_BITS_PER_XACT 8
69+
#define MXACT_MEMBER_FLAGS_PER_BYTE 1
70+
#define MXACT_MEMBER_XACT_BITMASK ((1 << MXACT_MEMBER_BITS_PER_XACT) - 1)
71+
72+
/* how many full bytes of flags are there in a group? */
73+
#define MULTIXACT_FLAGBYTES_PER_GROUP 4
74+
#define MULTIXACT_MEMBERS_PER_MEMBERGROUP \
75+
(MULTIXACT_FLAGBYTES_PER_GROUP * MXACT_MEMBER_FLAGS_PER_BYTE)
76+
/* size in bytes of a complete group */
77+
#define MULTIXACT_MEMBERGROUP_SIZE \
78+
(sizeof(TransactionId) * MULTIXACT_MEMBERS_PER_MEMBERGROUP + MULTIXACT_FLAGBYTES_PER_GROUP)
79+
#define MULTIXACT_MEMBERGROUPS_PER_PAGE (BLCKSZ / MULTIXACT_MEMBERGROUP_SIZE)
80+
#define MULTIXACT_MEMBERS_PER_PAGE \
81+
(MULTIXACT_MEMBERGROUPS_PER_PAGE * MULTIXACT_MEMBERS_PER_MEMBERGROUP)
82+
83+
/*
84+
* Because the number of items per page is not a divisor of the last item
85+
* number (member 0xFFFFFFFF), the last segment does not use the maximum number
86+
* of pages, and moreover the last used page therein does not use the same
87+
* number of items as previous pages. (Another way to say it is that the
88+
* 0xFFFFFFFF member is somewhere in the middle of the last page, so the page
89+
* has some empty space after that item.)
90+
*
91+
* This constant is the number of members in the last page of the last segment.
92+
*/
93+
#define MAX_MEMBERS_IN_LAST_MEMBERS_PAGE \
94+
((uint32) ((0xFFFFFFFF % MULTIXACT_MEMBERS_PER_PAGE) + 1))
95+
96+
/* page in which a member is to be found */
97+
static inline int64
98+
MXOffsetToMemberPage(MultiXactOffset offset)
99+
{
100+
return offset / MULTIXACT_MEMBERS_PER_PAGE;
101+
}
102+
103+
static inline int64
104+
MXOffsetToMemberSegment(MultiXactOffset offset)
105+
{
106+
return MXOffsetToMemberPage(offset) / SLRU_PAGES_PER_SEGMENT;
107+
}
108+
109+
/* Location (byte offset within page) of flag word for a given member */
110+
static inline int
111+
MXOffsetToFlagsOffset(MultiXactOffset offset)
112+
{
113+
MultiXactOffset group = offset / MULTIXACT_MEMBERS_PER_MEMBERGROUP;
114+
int grouponpg = group % MULTIXACT_MEMBERGROUPS_PER_PAGE;
115+
int byteoff = grouponpg * MULTIXACT_MEMBERGROUP_SIZE;
116+
117+
return byteoff;
118+
}
119+
120+
static inline int
121+
MXOffsetToFlagsBitShift(MultiXactOffset offset)
122+
{
123+
int member_in_group = offset % MULTIXACT_MEMBERS_PER_MEMBERGROUP;
124+
int bshift = member_in_group * MXACT_MEMBER_BITS_PER_XACT;
125+
126+
return bshift;
127+
}
128+
129+
/* Location (byte offset within page) of TransactionId of given member */
130+
static inline int
131+
MXOffsetToMemberOffset(MultiXactOffset offset)
132+
{
133+
int member_in_group = offset % MULTIXACT_MEMBERS_PER_MEMBERGROUP;
134+
135+
return MXOffsetToFlagsOffset(offset) +
136+
MULTIXACT_FLAGBYTES_PER_GROUP +
137+
member_in_group * sizeof(TransactionId);
138+
}
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140+
#endif /* MULTIXACT_INTERNAL_H */

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