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/*-------------------------------------------------------------------------
*
* expandeddatum.h
* Declarations for access to "expanded" value representations.
*
* Complex data types, particularly container types such as arrays and
* records, usually have on-disk representations that are compact but not
* especially convenient to modify. What's more, when we do modify them,
* having to recopy all the rest of the value can be extremely inefficient.
* Therefore, we provide a notion of an "expanded" representation that is used
* only in memory and is optimized more for computation than storage.
* The format appearing on disk is called the data type's "flattened"
* representation, since it is required to be a contiguous blob of bytes --
* but the type can have an expanded representation that is not. Data types
* must provide means to translate an expanded representation back to
* flattened form.
*
* An expanded object is meant to survive across multiple operations, but
* not to be enormously long-lived; for example it might be a local variable
* in a PL/pgSQL procedure. So its extra bulk compared to the on-disk format
* is a worthwhile trade-off.
*
* References to expanded objects are a type of TOAST pointer.
* Because of longstanding conventions in Postgres, this means that the
* flattened form of such an object must always be a varlena object.
* Fortunately that's no restriction in practice.
*
* There are actually two kinds of TOAST pointers for expanded objects:
* read-only and read-write pointers. Possession of one of the latter
* authorizes a function to modify the value in-place rather than copying it
* as would normally be required. Functions should always return a read-write
* pointer to any new expanded object they create. Functions that modify an
* argument value in-place must take care that they do not corrupt the old
* value if they fail partway through.
*
*
* Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* src/include/utils/expandeddatum.h
*
*-------------------------------------------------------------------------
*/
#ifndef EXPANDEDDATUM_H
#define EXPANDEDDATUM_H
/* Size of an EXTERNAL datum that contains a pointer to an expanded object */
#define EXPANDED_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(varatt_expanded))
/*
* "Methods" that must be provided for any expanded object.
*
* get_flat_size: compute space needed for flattened representation (total,
* including header).
*
* flatten_into: construct flattened representation in the caller-allocated
* space at *result, of size allocated_size (which will always be the result
* of a preceding get_flat_size call; it's passed for cross-checking).
*
* The flattened representation must be a valid in-line, non-compressed,
* 4-byte-header varlena object.
*
* Note: construction of a heap tuple from an expanded datum calls
* get_flat_size twice, so it's worthwhile to make sure that that doesn't
* incur too much overhead.
*/
typedef Size (*EOM_get_flat_size_method) (ExpandedObjectHeader *eohptr);
typedef void (*EOM_flatten_into_method) (ExpandedObjectHeader *eohptr,
void *result, Size allocated_size);
/* Struct of function pointers for an expanded object's methods */
typedef struct ExpandedObjectMethods
{
EOM_get_flat_size_method get_flat_size;
EOM_flatten_into_method flatten_into;
} ExpandedObjectMethods;
/*
* Every expanded object must contain this header; typically the header
* is embedded in some larger struct that adds type-specific fields.
*
* It is presumed that the header object and all subsidiary data are stored
* in eoh_context, so that the object can be freed by deleting that context,
* or its storage lifespan can be altered by reparenting the context.
* (In principle the object could own additional resources, such as malloc'd
* storage, and use a memory context reset callback to free them upon reset or
* deletion of eoh_context.)
*
* We set up two TOAST pointers within the standard header, one read-write
* and one read-only. This allows functions to return either kind of pointer
* without making an additional allocation, and in particular without worrying
* whether a separately palloc'd object would have sufficient lifespan.
* But note that these pointers are just a convenience; a pointer object
* appearing somewhere else would still be legal.
*
* The typedef declaration for this appears in postgres.h.
*/
struct ExpandedObjectHeader
{
/* Phony varlena header */
int32 vl_len_; /* always EOH_HEADER_MAGIC, see below */
/* Pointer to methods required for object type */
const ExpandedObjectMethods *eoh_methods;
/* Memory context containing this header and subsidiary data */
MemoryContext eoh_context;
/* Standard R/W TOAST pointer for this object is kept here */
char eoh_rw_ptr[EXPANDED_POINTER_SIZE];
/* Standard R/O TOAST pointer for this object is kept here */
char eoh_ro_ptr[EXPANDED_POINTER_SIZE];
};
/*
* Particularly for read-only functions, it is handy to be able to work with
* either regular "flat" varlena inputs or expanded inputs of the same data
* type. To allow determining which case an argument-fetching function has
* returned, the first int32 of an ExpandedObjectHeader always contains -1
* (EOH_HEADER_MAGIC to the code). This works since no 4-byte-header varlena
* could have that as its first 4 bytes. Caution: we could not reliably tell
* the difference between an ExpandedObjectHeader and a short-header object
* with this trick. However, it works fine if the argument fetching code
* always returns either a 4-byte-header flat object or an expanded object.
*/
#define EOH_HEADER_MAGIC (-1)
#define VARATT_IS_EXPANDED_HEADER(PTR) \
(((ExpandedObjectHeader *) (PTR))->vl_len_ == EOH_HEADER_MAGIC)
/*
* Generic support functions for expanded objects.
* (More of these might be worth inlining later.)
*/
#define EOHPGetRWDatum(eohptr) PointerGetDatum((eohptr)->eoh_rw_ptr)
#define EOHPGetRODatum(eohptr) PointerGetDatum((eohptr)->eoh_ro_ptr)
extern ExpandedObjectHeader *DatumGetEOHP(Datum d);
extern void EOH_init_header(ExpandedObjectHeader *eohptr,
const ExpandedObjectMethods *methods,
MemoryContext obj_context);
extern Size EOH_get_flat_size(ExpandedObjectHeader *eohptr);
extern void EOH_flatten_into(ExpandedObjectHeader *eohptr,
void *result, Size allocated_size);
extern bool DatumIsReadWriteExpandedObject(Datum d, bool isnull, int16 typlen);
extern Datum MakeExpandedObjectReadOnly(Datum d, bool isnull, int16 typlen);
extern Datum TransferExpandedObject(Datum d, MemoryContext new_parent);
extern void DeleteExpandedObject(Datum d);
#endif /* EXPANDEDDATUM_H */
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