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relfilelocator.h
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1 /*-------------------------------------------------------------------------
2  *
3  * relfilelocator.h
4  * Physical access information for relations.
5  *
6  *
7  * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
8  * Portions Copyright (c) 1994, Regents of the University of California
9  *
10  * src/include/storage/relfilelocator.h
11  *
12  *-------------------------------------------------------------------------
13  */
14 #ifndef RELFILELOCATOR_H
15 #define RELFILELOCATOR_H
16 
17 #include "common/relpath.h"
18 #include "storage/procnumber.h"
19 
20 /*
21  * RelFileLocator must provide all that we need to know to physically access
22  * a relation, with the exception of the backend's proc number, which can be
23  * provided separately. Note, however, that a "physical" relation is
24  * comprised of multiple files on the filesystem, as each fork is stored as
25  * a separate file, and each fork can be divided into multiple segments. See
26  * md.c.
27  *
28  * spcOid identifies the tablespace of the relation. It corresponds to
29  * pg_tablespace.oid.
30  *
31  * dbOid identifies the database of the relation. It is zero for
32  * "shared" relations (those common to all databases of a cluster).
33  * Nonzero dbOid values correspond to pg_database.oid.
34  *
35  * relNumber identifies the specific relation. relNumber corresponds to
36  * pg_class.relfilenode (NOT pg_class.oid, because we need to be able
37  * to assign new physical files to relations in some situations).
38  * Notice that relNumber is only unique within a database in a particular
39  * tablespace.
40  *
41  * Note: spcOid must be GLOBALTABLESPACE_OID if and only if dbOid is
42  * zero. We support shared relations only in the "global" tablespace.
43  *
44  * Note: in pg_class we allow reltablespace == 0 to denote that the
45  * relation is stored in its database's "default" tablespace (as
46  * identified by pg_database.dattablespace). However this shorthand
47  * is NOT allowed in RelFileLocator structs --- the real tablespace ID
48  * must be supplied when setting spcOid.
49  *
50  * Note: in pg_class, relfilenode can be zero to denote that the relation
51  * is a "mapped" relation, whose current true filenode number is available
52  * from relmapper.c. Again, this case is NOT allowed in RelFileLocators.
53  *
54  * Note: various places use RelFileLocator in hashtable keys. Therefore,
55  * there *must not* be any unused padding bytes in this struct. That
56  * should be safe as long as all the fields are of type Oid.
57  */
58 typedef struct RelFileLocator
59 {
60  Oid spcOid; /* tablespace */
61  Oid dbOid; /* database */
62  RelFileNumber relNumber; /* relation */
64 
65 /*
66  * Augmenting a relfilelocator with the backend's proc number provides all the
67  * information we need to locate the physical storage. 'backend' is
68  * INVALID_PROC_NUMBER for regular relations (those accessible to more than
69  * one backend), or the owning backend's proc number for backend-local
70  * relations. Backend-local relations are always transient and removed in
71  * case of a database crash; they are never WAL-logged or fsync'd.
72  */
73 typedef struct RelFileLocatorBackend
74 {
78 
79 #define RelFileLocatorBackendIsTemp(rlocator) \
80  ((rlocator).backend != INVALID_PROC_NUMBER)
81 
82 /*
83  * Note: RelFileLocatorEquals and RelFileLocatorBackendEquals compare relNumber
84  * first since that is most likely to be different in two unequal
85  * RelFileLocators. It is probably redundant to compare spcOid if the other
86  * fields are found equal, but do it anyway to be sure. Likewise for checking
87  * the backend number in RelFileLocatorBackendEquals.
88  */
89 #define RelFileLocatorEquals(locator1, locator2) \
90  ((locator1).relNumber == (locator2).relNumber && \
91  (locator1).dbOid == (locator2).dbOid && \
92  (locator1).spcOid == (locator2).spcOid)
93 
94 #define RelFileLocatorBackendEquals(locator1, locator2) \
95  ((locator1).locator.relNumber == (locator2).locator.relNumber && \
96  (locator1).locator.dbOid == (locator2).locator.dbOid && \
97  (locator1).backend == (locator2).backend && \
98  (locator1).locator.spcOid == (locator2).locator.spcOid)
99 
100 #endif /* RELFILELOCATOR_H */
unsigned int Oid
Definition: postgres_ext.h:31
int ProcNumber
Definition: procnumber.h:24
struct RelFileLocator RelFileLocator
struct RelFileLocatorBackend RelFileLocatorBackend
Oid RelFileNumber
Definition: relpath.h:25
RelFileLocator locator
RelFileNumber relNumber