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290 lines
11 KiB
C
290 lines
11 KiB
C
/*-------------------------------------------------------------------------
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*
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* sortsupport.h
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* Framework for accelerated sorting.
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*
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* Traditionally, PostgreSQL has implemented sorting by repeatedly invoking
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* an SQL-callable comparison function "cmp(x, y) returns int" on pairs of
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* values to be compared, where the comparison function is the BTORDER_PROC
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* pg_amproc support function of the appropriate btree index opclass.
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*
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* This file defines alternative APIs that allow sorting to be performed with
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* reduced overhead. To support lower-overhead sorting, a btree opclass may
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* provide a BTSORTSUPPORT_PROC pg_amproc entry, which must take a single
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* argument of type internal and return void. The argument is actually a
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* pointer to a SortSupportData struct, which is defined below.
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*
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* If provided, the BTSORTSUPPORT function will be called during sort setup,
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* and it must initialize the provided struct with pointers to function(s)
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* that can be called to perform sorting. This API is defined to allow
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* multiple acceleration mechanisms to be supported, but no opclass is
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* required to provide all of them. The BTSORTSUPPORT function should
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* simply not set any function pointers for mechanisms it doesn't support.
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* Opclasses that provide BTSORTSUPPORT and don't provide a comparator
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* function will have a shim set up by sort support automatically. However,
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* opclasses that support the optional additional abbreviated key capability
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* must always provide an authoritative comparator used to tie-break
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* inconclusive abbreviated comparisons and also used when aborting
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* abbreviation. Furthermore, a converter and abort/costing function must be
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* provided.
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*
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* All sort support functions will be passed the address of the
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* SortSupportData struct when called, so they can use it to store
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* additional private data as needed. In particular, for collation-aware
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* datatypes, the ssup_collation field is set before calling BTSORTSUPPORT
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* and is available to all support functions. Additional opclass-dependent
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* data can be stored using the ssup_extra field. Any such data
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* should be allocated in the ssup_cxt memory context.
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*
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* Note: since pg_amproc functions are indexed by (lefttype, righttype)
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* it is possible to associate a BTSORTSUPPORT function with a cross-type
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* comparison. This could sensibly be used to provide a fast comparator
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* function for such cases, but probably not any other acceleration method.
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*
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*
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* Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* src/include/utils/sortsupport.h
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*
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*-------------------------------------------------------------------------
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*/
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#ifndef SORTSUPPORT_H
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#define SORTSUPPORT_H
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#include "access/attnum.h"
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#include "utils/relcache.h"
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typedef struct SortSupportData *SortSupport;
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typedef struct SortSupportData
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{
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/*
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* These fields are initialized before calling the BTSORTSUPPORT function
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* and should not be changed later.
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*/
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MemoryContext ssup_cxt; /* Context containing sort info */
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Oid ssup_collation; /* Collation to use, or InvalidOid */
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/*
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* Additional sorting parameters; but unlike ssup_collation, these can be
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* changed after BTSORTSUPPORT is called, so don't use them in selecting
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* sort support functions.
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*/
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bool ssup_reverse; /* descending-order sort? */
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bool ssup_nulls_first; /* sort nulls first? */
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/*
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* These fields are workspace for callers, and should not be touched by
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* opclass-specific functions.
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*/
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AttrNumber ssup_attno; /* column number to sort */
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/*
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* ssup_extra is zeroed before calling the BTSORTSUPPORT function, and is
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* not touched subsequently by callers.
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*/
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void *ssup_extra; /* Workspace for opclass functions */
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/*
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* Function pointers are zeroed before calling the BTSORTSUPPORT function,
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* and must be set by it for any acceleration methods it wants to supply.
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* The comparator pointer must be set, others are optional.
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*/
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/*
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* Comparator function has the same API as the traditional btree
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* comparison function, ie, return <0, 0, or >0 according as x is less
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* than, equal to, or greater than y. Note that x and y are guaranteed
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* not null, and there is no way to return null either. Do not return
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* INT_MIN, as callers are allowed to negate the result before using it.
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*
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* This may be either the authoritative comparator, or the abbreviated
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* comparator. Core code may switch this over the initial preference of an
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* opclass support function despite originally indicating abbreviation was
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* applicable, by assigning the authoritative comparator back.
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*/
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int (*comparator) (Datum x, Datum y, SortSupport ssup);
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/*
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* "Abbreviated key" infrastructure follows.
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*
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* All callbacks must be set by sortsupport opclasses that make use of this
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* optional additional infrastructure (unless for whatever reasons the
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* opclass doesn't proceed with abbreviation, in which case
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* abbrev_converter must not be set).
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*
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* This allows opclass authors to supply a conversion routine, used to
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* create an alternative representation of the underlying type (an
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* "abbreviated key"). Typically, this representation is an ad-hoc,
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* pass-by-value Datum format that only the opclass has knowledge of. An
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* alternative comparator, used only with this alternative representation
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* must also be provided (which is assigned to "comparator"). This
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* representation is a simple approximation of the original Datum. It must
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* be possible to compare datums of this representation with each other
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* using the supplied alternative comparator, and have any non-zero return
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* value be a reliable proxy for what a proper comparison would indicate.
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* Returning zero from the alternative comparator does not indicate
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* equality, as with a conventional support routine 1, though -- it
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* indicates that it wasn't possible to determine how the two abbreviated
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* values compared. A proper comparison, using "abbrev_full_comparator"/
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* ApplySortAbbrevFullComparator() is therefore required. In many cases
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* this results in most or all comparisons only using the cheap alternative
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* comparison func, which is typically implemented as code that compiles to
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* just a few CPU instructions. CPU cache miss penalties are expensive; to
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* get good overall performance, sort infrastructure must heavily weigh
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* cache performance.
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*
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* Opclass authors must consider the final cardinality of abbreviated keys
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* when devising an encoding scheme. It's possible for a strategy to work
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* better than an alternative strategy with one usage pattern, while the
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* reverse might be true for another usage pattern. All of these factors
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* must be considered.
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*/
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/*
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* "abbreviate" concerns whether or not the abbreviated key optimization is
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* applicable in principle (that is, the sortsupport routine needs to know
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* if its dealing with a key where an abbreviated representation can
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* usefully be packed together. Conventionally, this is the leading
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* attribute key). Note, however, that in order to determine that
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* abbreviation is not in play, the core code always checks whether or not
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* the opclass has set abbrev_converter. This is a one way, one time
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* message to the opclass.
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*/
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bool abbreviate;
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/*
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* Converter to abbreviated format, from original representation. Core
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* code uses this callback to convert from a pass-by-reference "original"
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* Datum to a pass-by-value abbreviated key Datum. Note that original is
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* guaranteed NOT NULL, because it doesn't make sense to factor NULLness
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* into ad-hoc cost model.
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*
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* abbrev_converter is tested to see if abbreviation is in play. Core code
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* may set it to NULL to indicate abbreviation should not be used (which is
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* something sortsupport routines need not concern themselves with).
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* However, sortsupport routines must not set it when it is immediately
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* established that abbreviation should not proceed (e.g., for !abbreviate
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* calls, or due to platform-specific impediments to using abbreviation).
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*/
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Datum (*abbrev_converter) (Datum original, SortSupport ssup);
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/*
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* abbrev_abort callback allows clients to verify that the current strategy
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* is working out, using a sortsupport routine defined ad-hoc cost model.
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* If there is a lot of duplicate abbreviated keys in practice, it's useful
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* to be able to abandon the strategy before paying too high a cost in
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* conversion (perhaps certain opclass-specific adaptations are useful
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* too).
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*/
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bool (*abbrev_abort) (int memtupcount, SortSupport ssup);
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/*
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* Full, authoritative comparator for key that an abbreviated
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* representation was generated for, used when an abbreviated comparison
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* was inconclusive (by calling ApplySortComparatorFull()), or used to
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* replace "comparator" when core system ultimately decides against
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* abbreviation.
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*/
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int (*abbrev_full_comparator) (Datum x, Datum y, SortSupport ssup);
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} SortSupportData;
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/*
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* ApplySortComparator should be inlined if possible. See STATIC_IF_INLINE
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* in c.h.
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*/
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#ifndef PG_USE_INLINE
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extern int ApplySortComparator(Datum datum1, bool isNull1,
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Datum datum2, bool isNull2,
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SortSupport ssup);
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extern int ApplySortAbbrevFullComparator(Datum datum1, bool isNull1,
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Datum datum2, bool isNull2,
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SortSupport ssup);
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#endif /* !PG_USE_INLINE */
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#if defined(PG_USE_INLINE) || defined(SORTSUPPORT_INCLUDE_DEFINITIONS)
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/*
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* Apply a sort comparator function and return a 3-way comparison result.
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* This takes care of handling reverse-sort and NULLs-ordering properly.
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*/
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STATIC_IF_INLINE int
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ApplySortComparator(Datum datum1, bool isNull1,
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Datum datum2, bool isNull2,
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SortSupport ssup)
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{
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int compare;
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if (isNull1)
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{
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if (isNull2)
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compare = 0; /* NULL "=" NULL */
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else if (ssup->ssup_nulls_first)
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compare = -1; /* NULL "<" NOT_NULL */
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else
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compare = 1; /* NULL ">" NOT_NULL */
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}
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else if (isNull2)
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{
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if (ssup->ssup_nulls_first)
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compare = 1; /* NOT_NULL ">" NULL */
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else
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compare = -1; /* NOT_NULL "<" NULL */
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}
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else
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{
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compare = (*ssup->comparator) (datum1, datum2, ssup);
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if (ssup->ssup_reverse)
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compare = -compare;
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}
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return compare;
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}
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/*
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* Apply a sort comparator function and return a 3-way comparison using full,
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* authoritative comparator. This takes care of handling reverse-sort and
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* NULLs-ordering properly.
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*/
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STATIC_IF_INLINE int
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ApplySortAbbrevFullComparator(Datum datum1, bool isNull1,
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Datum datum2, bool isNull2,
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SortSupport ssup)
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{
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int compare;
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if (isNull1)
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{
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if (isNull2)
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compare = 0; /* NULL "=" NULL */
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else if (ssup->ssup_nulls_first)
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compare = -1; /* NULL "<" NOT_NULL */
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else
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compare = 1; /* NULL ">" NOT_NULL */
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}
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else if (isNull2)
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{
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if (ssup->ssup_nulls_first)
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compare = 1; /* NOT_NULL ">" NULL */
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else
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compare = -1; /* NOT_NULL "<" NULL */
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}
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else
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{
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compare = (*ssup->abbrev_full_comparator) (datum1, datum2, ssup);
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if (ssup->ssup_reverse)
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compare = -compare;
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}
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return compare;
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}
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#endif /*-- PG_USE_INLINE || SORTSUPPORT_INCLUDE_DEFINITIONS */
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/* Other functions in utils/sort/sortsupport.c */
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extern void PrepareSortSupportComparisonShim(Oid cmpFunc, SortSupport ssup);
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extern void PrepareSortSupportFromOrderingOp(Oid orderingOp, SortSupport ssup);
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extern void PrepareSortSupportFromIndexRel(Relation indexRel, int16 strategy,
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SortSupport ssup);
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#endif /* SORTSUPPORT_H */
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