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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. * * 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) INVERT_COMPARE_RESULT(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) INVERT_COMPARE_RESULT(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 */