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/*-------------------------------------------------------------------------
*
* sortsupport.h
* Framework for accelerated sorting.
*
* Traditionally, PostgreSQL has implemented sorting by repeatedly invoking
* an SQL-callable comparison function "cmp(x, y) returns int" on pairs of
* values to be compared, where the comparison function is the BTORDER_PROC
* pg_amproc support function of the appropriate btree index opclass.
*
* This file defines alternative APIs that allow sorting to be performed with
* reduced overhead. To support lower-overhead sorting, a btree opclass may
* provide a BTSORTSUPPORT_PROC pg_amproc entry, which must take a single
* argument of type internal and return void. The argument is actually a
* pointer to a SortSupportData struct, which is defined below.
*
* If provided, the BTSORTSUPPORT function will be called during sort setup,
* and it must initialize the provided struct with pointers to function(s)
* that can be called to perform sorting. This API is defined to allow
* multiple acceleration mechanisms to be supported, but no opclass is
* required to provide all of them. The BTSORTSUPPORT function should
* simply not set any function pointers for mechanisms it doesn't support.
* Opclasses that provide BTSORTSUPPORT and don't provide a comparator
* function will have a shim set up by sort support automatically. However,
* opclasses that support the optional additional abbreviated key capability
* must always provide an authoritative comparator used to tie-break
* inconclusive abbreviated comparisons and also used when aborting
* abbreviation. Furthermore, a converter and abort/costing function must be
* provided.
*
* All sort support functions will be passed the address of the
* SortSupportData struct when called, so they can use it to store
* additional private data as needed. In particular, for collation-aware
* datatypes, the ssup_collation field is set before calling BTSORTSUPPORT
* and is available to all support functions. Additional opclass-dependent
* data can be stored using the ssup_extra field. Any such data
* should be allocated in the ssup_cxt memory context.
*
* Note: since pg_amproc functions are indexed by (lefttype, righttype)
* it is possible to associate a BTSORTSUPPORT function with a cross-type
* comparison. This could sensibly be used to provide a fast comparator
* function for such cases, but probably not any other acceleration method.
*
*
* Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* src/include/utils/sortsupport.h
*
*-------------------------------------------------------------------------
*/
#ifndef SORTSUPPORT_H
#define SORTSUPPORT_H
#include "access/attnum.h"
#include "utils/relcache.h"
typedef struct SortSupportData *SortSupport;
typedef struct SortSupportData
{
/*
* These fields are initialized before calling the BTSORTSUPPORT function
* and should not be changed later.
*/
MemoryContext ssup_cxt; /* Context containing sort info */
Oid ssup_collation; /* Collation to use, or InvalidOid */
/*
* Additional sorting parameters; but unlike ssup_collation, these can be
* changed after BTSORTSUPPORT is called, so don't use them in selecting
* sort support functions.
*/
bool ssup_reverse; /* descending-order sort? */
bool ssup_nulls_first; /* sort nulls first? */
/*
* These fields are workspace for callers, and should not be touched by
* opclass-specific functions.
*/
AttrNumber ssup_attno; /* column number to sort */
/*
* ssup_extra is zeroed before calling the BTSORTSUPPORT function, and is
* not touched subsequently by callers.
*/
void *ssup_extra; /* Workspace for opclass functions */
/*
* Function pointers are zeroed before calling the BTSORTSUPPORT function,
* and must be set by it for any acceleration methods it wants to supply.
* The comparator pointer must be set, others are optional.
*/
/*
* Comparator function has the same API as the traditional btree
* comparison function, ie, return <0, 0, or >0 according as x is less
* than, equal to, or greater than y. Note that x and y are guaranteed
* not null, and there is no way to return null either. Do not return
* INT_MIN, as callers are allowed to negate the result before using it.
*
* This may be either the authoritative comparator, or the abbreviated
* comparator. Core code may switch this over the initial preference of
* an opclass support function despite originally indicating abbreviation
* was applicable, by assigning the authoritative comparator back.
*/
int (*comparator) (Datum x, Datum y, SortSupport ssup);
/*
* "Abbreviated key" infrastructure follows.
*
* All callbacks must be set by sortsupport opclasses that make use of
* this optional additional infrastructure (unless for whatever reasons
* the opclass doesn't proceed with abbreviation, in which case
* abbrev_converter must not be set).
*
* This allows opclass authors to supply a conversion routine, used to
* create an alternative representation of the underlying type (an
* "abbreviated key"). This representation must be pass-by-value and
* typically will use some ad-hoc format that only the opclass has
* knowledge of. An alternative comparator, used only with this
* alternative representation must also be provided (which is assigned to
* "comparator"). This representation is a simple approximation of the
* original Datum. It must be possible to compare datums of this
* representation with each other using the supplied alternative
* comparator, and have any non-zero return value be a reliable proxy for
* what a proper comparison would indicate. Returning zero from the
* alternative comparator does not indicate equality, as with a
* conventional support routine 1, though -- it indicates that it wasn't
* possible to determine how the two abbreviated values compared. A
* proper comparison, using "abbrev_full_comparator"/
* ApplySortAbbrevFullComparator() is therefore required. In many cases
* this results in most or all comparisons only using the cheap
* alternative comparison func, which is typically implemented as code
* that compiles to just a few CPU instructions. CPU cache miss penalties
* are expensive; to get good overall performance, sort infrastructure
* must heavily weigh cache performance.
*
* Opclass authors must consider the final cardinality of abbreviated keys
* when devising an encoding scheme. It's possible for a strategy to work
* better than an alternative strategy with one usage pattern, while the
* reverse might be true for another usage pattern. All of these factors
* must be considered.
*/
/*
* "abbreviate" concerns whether or not the abbreviated key optimization
* is applicable in principle (that is, the sortsupport routine needs to
* know if its dealing with a key where an abbreviated representation can
* usefully be packed together. Conventionally, this is the leading
* attribute key). Note, however, that in order to determine that
* abbreviation is not in play, the core code always checks whether or not
* the opclass has set abbrev_converter. This is a one way, one time
* message to the opclass.
*/
bool abbreviate;
/*
* Converter to abbreviated format, from original representation. Core
* code uses this callback to convert from a pass-by-reference "original"
* Datum to a pass-by-value abbreviated key Datum. Note that original is
* guaranteed NOT NULL, because it doesn't make sense to factor NULLness
* into ad-hoc cost model.
*
* abbrev_converter is tested to see if abbreviation is in play. Core
* code may set it to NULL to indicate abbreviation should not be used
* (which is something sortsupport routines need not concern themselves
* with). However, sortsupport routines must not set it when it is
* immediately established that abbreviation should not proceed (e.g., for
* !abbreviate calls, or due to platform-specific impediments to using
* abbreviation).
*/
Datum (*abbrev_converter) (Datum original, SortSupport ssup);
/*
* abbrev_abort callback allows clients to verify that the current
* strategy is working out, using a sortsupport routine defined ad-hoc
* cost model. If there is a lot of duplicate abbreviated keys in
* practice, it's useful to be able to abandon the strategy before paying
* too high a cost in conversion (perhaps certain opclass-specific
* adaptations are useful too).
*/
bool (*abbrev_abort) (int memtupcount, SortSupport ssup);
/*
* Full, authoritative comparator for key that an abbreviated
* representation was generated for, used when an abbreviated comparison
* was inconclusive (by calling ApplySortComparatorFull()), or used to
* replace "comparator" when core system ultimately decides against
* abbreviation.
*/
int (*abbrev_full_comparator) (Datum x, Datum y, SortSupport ssup);
} SortSupportData;
/*
* Apply a sort comparator function and return a 3-way comparison result.
* This takes care of handling reverse-sort and NULLs-ordering properly.
*/
static inline int
ApplySortComparator(Datum datum1, bool isNull1,
Datum datum2, bool isNull2,
SortSupport ssup)
{
int compare;
if (isNull1)
{
if (isNull2)
compare = 0; /* NULL "=" NULL */
else if (ssup->ssup_nulls_first)
compare = -1; /* NULL "<" NOT_NULL */
else
compare = 1; /* NULL ">" NOT_NULL */
}
else if (isNull2)
{
if (ssup->ssup_nulls_first)
compare = 1; /* NOT_NULL ">" NULL */
else
compare = -1; /* NOT_NULL "<" NULL */
}
else
{
compare = (*ssup->comparator) (datum1, datum2, ssup);
if (ssup->ssup_reverse)
compare = -compare;
}
return compare;
}
/*
* Apply a sort comparator function and return a 3-way comparison using full,
* authoritative comparator. This takes care of handling reverse-sort and
* NULLs-ordering properly.
*/
static inline int
ApplySortAbbrevFullComparator(Datum datum1, bool isNull1,
Datum datum2, bool isNull2,
SortSupport ssup)
{
int compare;
if (isNull1)
{
if (isNull2)
compare = 0; /* NULL "=" NULL */
else if (ssup->ssup_nulls_first)
compare = -1; /* NULL "<" NOT_NULL */
else
compare = 1; /* NULL ">" NOT_NULL */
}
else if (isNull2)
{
if (ssup->ssup_nulls_first)
compare = 1; /* NOT_NULL ">" NULL */
else
compare = -1; /* NOT_NULL "<" NULL */
}
else
{
compare = (*ssup->abbrev_full_comparator) (datum1, datum2, ssup);
if (ssup->ssup_reverse)
compare = -compare;
}
return compare;
}
/* Other functions in utils/sort/sortsupport.c */
extern void PrepareSortSupportComparisonShim(Oid cmpFunc, SortSupport ssup);
extern void PrepareSortSupportFromOrderingOp(Oid orderingOp, SortSupport ssup);
extern void PrepareSortSupportFromIndexRel(Relation indexRel, int16 strategy,
SortSupport ssup);
#endif /* SORTSUPPORT_H */
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