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/*
* multixact_internal.h
*
* PostgreSQL multi-transaction-log manager internal declarations
*
* These functions and definitions are for dealing with pg_multixact SLRU
* pages. They are internal to multixact.c, but they are exported here to
* allow pg_upgrade to write pg_multixact files directly.
*
* Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* src/include/access/multixact_internal.h
*/
#ifndef MULTIXACT_INTERNAL_H
/*
* Note: This is not only to prevent including this file twice.
* MULTIXACT_INTERNAL_H is checked explicitly in multixact_read_v18.c.
*/
#define MULTIXACT_INTERNAL_H
#include "access/multixact.h"
/*
* Defines for MultiXactOffset page sizes. A page is the same BLCKSZ as is
* used everywhere else in Postgres.
*/
/* We need 8 bytes per offset */
#define MULTIXACT_OFFSETS_PER_PAGE (BLCKSZ / sizeof(MultiXactOffset))
static inline int64
MultiXactIdToOffsetPage(MultiXactId multi)
{
return multi / MULTIXACT_OFFSETS_PER_PAGE;
}
static inline int
MultiXactIdToOffsetEntry(MultiXactId multi)
{
return multi % MULTIXACT_OFFSETS_PER_PAGE;
}
static inline int64
MultiXactIdToOffsetSegment(MultiXactId multi)
{
return MultiXactIdToOffsetPage(multi) / SLRU_PAGES_PER_SEGMENT;
}
/*
* The situation for members is a bit more complex: we store one byte of
* additional flag bits for each TransactionId. To do this without getting
* into alignment issues, we store four bytes of flags, and then the
* corresponding 4 Xids. Each such 5-word (20-byte) set we call a "group", and
* are stored as a whole in pages. Thus, with 8kB BLCKSZ, we keep 409 groups
* per page. This wastes 12 bytes per page, but that's OK -- simplicity (and
* performance) trumps space efficiency here.
*
* Note that the "offset" macros work with byte offset, not array indexes, so
* arithmetic must be done using "char *" pointers.
*/
/* We need eight bits per xact, so one xact fits in a byte */
#define MXACT_MEMBER_BITS_PER_XACT 8
#define MXACT_MEMBER_FLAGS_PER_BYTE 1
#define MXACT_MEMBER_XACT_BITMASK ((1 << MXACT_MEMBER_BITS_PER_XACT) - 1)
/* how many full bytes of flags are there in a group? */
#define MULTIXACT_FLAGBYTES_PER_GROUP 4
#define MULTIXACT_MEMBERS_PER_MEMBERGROUP \
(MULTIXACT_FLAGBYTES_PER_GROUP * MXACT_MEMBER_FLAGS_PER_BYTE)
/* size in bytes of a complete group */
#define MULTIXACT_MEMBERGROUP_SIZE \
(sizeof(TransactionId) * MULTIXACT_MEMBERS_PER_MEMBERGROUP + MULTIXACT_FLAGBYTES_PER_GROUP)
#define MULTIXACT_MEMBERGROUPS_PER_PAGE (BLCKSZ / MULTIXACT_MEMBERGROUP_SIZE)
#define MULTIXACT_MEMBERS_PER_PAGE \
(MULTIXACT_MEMBERGROUPS_PER_PAGE * MULTIXACT_MEMBERS_PER_MEMBERGROUP)
/* page in which a member is to be found */
static inline int64
MXOffsetToMemberPage(MultiXactOffset offset)
{
return offset / MULTIXACT_MEMBERS_PER_PAGE;
}
static inline int64
MXOffsetToMemberSegment(MultiXactOffset offset)
{
return MXOffsetToMemberPage(offset) / SLRU_PAGES_PER_SEGMENT;
}
/* Location (byte offset within page) of flag word for a given member */
static inline int
MXOffsetToFlagsOffset(MultiXactOffset offset)
{
MultiXactOffset group = offset / MULTIXACT_MEMBERS_PER_MEMBERGROUP;
int grouponpg = group % MULTIXACT_MEMBERGROUPS_PER_PAGE;
int byteoff = grouponpg * MULTIXACT_MEMBERGROUP_SIZE;
return byteoff;
}
static inline int
MXOffsetToFlagsBitShift(MultiXactOffset offset)
{
int member_in_group = offset % MULTIXACT_MEMBERS_PER_MEMBERGROUP;
int bshift = member_in_group * MXACT_MEMBER_BITS_PER_XACT;
return bshift;
}
/* Location (byte offset within page) of TransactionId of given member */
static inline int
MXOffsetToMemberOffset(MultiXactOffset offset)
{
int member_in_group = offset % MULTIXACT_MEMBERS_PER_MEMBERGROUP;
return MXOffsetToFlagsOffset(offset) +
MULTIXACT_FLAGBYTES_PER_GROUP +
member_in_group * sizeof(TransactionId);
}
#endif /* MULTIXACT_INTERNAL_H */
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