TLA Line data Source code
1 : /*-------------------------------------------------------------------------
2 : *
3 : * expandeddatum.h
4 : * Declarations for access to "expanded" value representations.
5 : *
6 : * Complex data types, particularly container types such as arrays and
7 : * records, usually have on-disk representations that are compact but not
8 : * especially convenient to modify. What's more, when we do modify them,
9 : * having to recopy all the rest of the value can be extremely inefficient.
10 : * Therefore, we provide a notion of an "expanded" representation that is used
11 : * only in memory and is optimized more for computation than storage.
12 : * The format appearing on disk is called the data type's "flattened"
13 : * representation, since it is required to be a contiguous blob of bytes --
14 : * but the type can have an expanded representation that is not. Data types
15 : * must provide means to translate an expanded representation back to
16 : * flattened form.
17 : *
18 : * An expanded object is meant to survive across multiple operations, but
19 : * not to be enormously long-lived; for example it might be a local variable
20 : * in a PL/pgSQL procedure. So its extra bulk compared to the on-disk format
21 : * is a worthwhile trade-off.
22 : *
23 : * References to expanded objects are a type of TOAST pointer.
24 : * Because of longstanding conventions in Postgres, this means that the
25 : * flattened form of such an object must always be a varlena object.
26 : * Fortunately that's no restriction in practice.
27 : *
28 : * There are actually two kinds of TOAST pointers for expanded objects:
29 : * read-only and read-write pointers. Possession of one of the latter
30 : * authorizes a function to modify the value in-place rather than copying it
31 : * as would normally be required. Functions should always return a read-write
32 : * pointer to any new expanded object they create. Functions that modify an
33 : * argument value in-place must take care that they do not corrupt the old
34 : * value if they fail partway through.
35 : *
36 : *
37 : * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
38 : * Portions Copyright (c) 1994, Regents of the University of California
39 : *
40 : * src/include/utils/expandeddatum.h
41 : *
42 : *-------------------------------------------------------------------------
43 : */
44 : #ifndef EXPANDEDDATUM_H
45 : #define EXPANDEDDATUM_H
46 :
47 : #include "varatt.h"
48 :
49 : /* Size of an EXTERNAL datum that contains a pointer to an expanded object */
50 : #define EXPANDED_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(varatt_expanded))
51 :
52 : /*
53 : * "Methods" that must be provided for any expanded object.
54 : *
55 : * get_flat_size: compute space needed for flattened representation (total,
56 : * including header).
57 : *
58 : * flatten_into: construct flattened representation in the caller-allocated
59 : * space at *result, of size allocated_size (which will always be the result
60 : * of a preceding get_flat_size call; it's passed for cross-checking).
61 : *
62 : * The flattened representation must be a valid in-line, non-compressed,
63 : * 4-byte-header varlena object.
64 : *
65 : * Note: construction of a heap tuple from an expanded datum calls
66 : * get_flat_size twice, so it's worthwhile to make sure that that doesn't
67 : * incur too much overhead.
68 : */
69 : typedef Size (*EOM_get_flat_size_method) (ExpandedObjectHeader *eohptr);
70 : typedef void (*EOM_flatten_into_method) (ExpandedObjectHeader *eohptr,
71 : void *result, Size allocated_size);
72 :
73 : /* Struct of function pointers for an expanded object's methods */
74 : typedef struct ExpandedObjectMethods
75 : {
76 : EOM_get_flat_size_method get_flat_size;
77 : EOM_flatten_into_method flatten_into;
78 : } ExpandedObjectMethods;
79 :
80 : /*
81 : * Every expanded object must contain this header; typically the header
82 : * is embedded in some larger struct that adds type-specific fields.
83 : *
84 : * It is presumed that the header object and all subsidiary data are stored
85 : * in eoh_context, so that the object can be freed by deleting that context,
86 : * or its storage lifespan can be altered by reparenting the context.
87 : * (In principle the object could own additional resources, such as malloc'd
88 : * storage, and use a memory context reset callback to free them upon reset or
89 : * deletion of eoh_context.)
90 : *
91 : * We set up two TOAST pointers within the standard header, one read-write
92 : * and one read-only. This allows functions to return either kind of pointer
93 : * without making an additional allocation, and in particular without worrying
94 : * whether a separately palloc'd object would have sufficient lifespan.
95 : * But note that these pointers are just a convenience; a pointer object
96 : * appearing somewhere else would still be legal.
97 : *
98 : * The typedef declaration for this appears in postgres.h.
99 : */
100 : struct ExpandedObjectHeader
101 : {
102 : /* Phony varlena header */
103 : int32 vl_len_; /* always EOH_HEADER_MAGIC, see below */
104 :
105 : /* Pointer to methods required for object type */
106 : const ExpandedObjectMethods *eoh_methods;
107 :
108 : /* Memory context containing this header and subsidiary data */
109 : MemoryContext eoh_context;
110 :
111 : /* Standard R/W TOAST pointer for this object is kept here */
112 : char eoh_rw_ptr[EXPANDED_POINTER_SIZE];
113 :
114 : /* Standard R/O TOAST pointer for this object is kept here */
115 : char eoh_ro_ptr[EXPANDED_POINTER_SIZE];
116 : };
117 :
118 : /*
119 : * Particularly for read-only functions, it is handy to be able to work with
120 : * either regular "flat" varlena inputs or expanded inputs of the same data
121 : * type. To allow determining which case an argument-fetching function has
122 : * returned, the first int32 of an ExpandedObjectHeader always contains -1
123 : * (EOH_HEADER_MAGIC to the code). This works since no 4-byte-header varlena
124 : * could have that as its first 4 bytes. Caution: we could not reliably tell
125 : * the difference between an ExpandedObjectHeader and a short-header object
126 : * with this trick. However, it works fine if the argument fetching code
127 : * always returns either a 4-byte-header flat object or an expanded object.
128 : */
129 : #define EOH_HEADER_MAGIC (-1)
130 : #define VARATT_IS_EXPANDED_HEADER(PTR) \
131 : (((varattrib_4b *) (PTR))->va_4byte.va_header == (uint32) EOH_HEADER_MAGIC)
132 :
133 : /*
134 : * Generic support functions for expanded objects.
135 : * (More of these might be worth inlining later.)
136 : */
137 :
138 : static inline Datum
139 GNC 20764 : EOHPGetRWDatum(const struct ExpandedObjectHeader *eohptr)
140 : {
141 20764 : return PointerGetDatum(eohptr->eoh_rw_ptr);
142 : }
143 :
144 : static inline Datum
145 15392 : EOHPGetRODatum(const struct ExpandedObjectHeader *eohptr)
146 : {
147 15392 : return PointerGetDatum(eohptr->eoh_ro_ptr);
148 : }
149 :
150 ECB : /* Does the Datum represent a writable expanded object? */
151 : #define DatumIsReadWriteExpandedObject(d, isnull, typlen) \
152 : (((isnull) || (typlen) != -1) ? false : \
153 : VARATT_IS_EXTERNAL_EXPANDED_RW(DatumGetPointer(d)))
154 :
155 : #define MakeExpandedObjectReadOnly(d, isnull, typlen) \
156 : (((isnull) || (typlen) != -1) ? (d) : \
157 : MakeExpandedObjectReadOnlyInternal(d))
158 :
159 : extern ExpandedObjectHeader *DatumGetEOHP(Datum d);
160 : extern void EOH_init_header(ExpandedObjectHeader *eohptr,
161 : const ExpandedObjectMethods *methods,
162 : MemoryContext obj_context);
163 : extern Size EOH_get_flat_size(ExpandedObjectHeader *eohptr);
164 : extern void EOH_flatten_into(ExpandedObjectHeader *eohptr,
165 : void *result, Size allocated_size);
166 : extern Datum MakeExpandedObjectReadOnlyInternal(Datum d);
167 : extern Datum TransferExpandedObject(Datum d, MemoryContext new_parent);
168 : extern void DeleteExpandedObject(Datum d);
169 :
170 : #endif /* EXPANDEDDATUM_H */
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