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sortsupport.h
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1 /*-------------------------------------------------------------------------
2  *
3  * sortsupport.h
4  * Framework for accelerated sorting.
5  *
6  * Traditionally, PostgreSQL has implemented sorting by repeatedly invoking
7  * an SQL-callable comparison function "cmp(x, y) returns int" on pairs of
8  * values to be compared, where the comparison function is the BTORDER_PROC
9  * pg_amproc support function of the appropriate btree index opclass.
10  *
11  * This file defines alternative APIs that allow sorting to be performed with
12  * reduced overhead. To support lower-overhead sorting, a btree opclass may
13  * provide a BTSORTSUPPORT_PROC pg_amproc entry, which must take a single
14  * argument of type internal and return void. The argument is actually a
15  * pointer to a SortSupportData struct, which is defined below.
16  *
17  * If provided, the BTSORTSUPPORT function will be called during sort setup,
18  * and it must initialize the provided struct with pointers to function(s)
19  * that can be called to perform sorting. This API is defined to allow
20  * multiple acceleration mechanisms to be supported, but no opclass is
21  * required to provide all of them. The BTSORTSUPPORT function should
22  * simply not set any function pointers for mechanisms it doesn't support.
23  * Opclasses that provide BTSORTSUPPORT and don't provide a comparator
24  * function will have a shim set up by sort support automatically. However,
25  * opclasses that support the optional additional abbreviated key capability
26  * must always provide an authoritative comparator used to tie-break
27  * inconclusive abbreviated comparisons and also used when aborting
28  * abbreviation. Furthermore, a converter and abort/costing function must be
29  * provided.
30  *
31  * All sort support functions will be passed the address of the
32  * SortSupportData struct when called, so they can use it to store
33  * additional private data as needed. In particular, for collation-aware
34  * datatypes, the ssup_collation field is set before calling BTSORTSUPPORT
35  * and is available to all support functions. Additional opclass-dependent
36  * data can be stored using the ssup_extra field. Any such data
37  * should be allocated in the ssup_cxt memory context.
38  *
39  * Note: since pg_amproc functions are indexed by (lefttype, righttype)
40  * it is possible to associate a BTSORTSUPPORT function with a cross-type
41  * comparison. This could sensibly be used to provide a fast comparator
42  * function for such cases, but probably not any other acceleration method.
43  *
44  *
45  * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
46  * Portions Copyright (c) 1994, Regents of the University of California
47  *
48  * src/include/utils/sortsupport.h
49  *
50  *-------------------------------------------------------------------------
51  */
52 #ifndef SORTSUPPORT_H
53 #define SORTSUPPORT_H
54 
55 #include "access/attnum.h"
56 #include "utils/relcache.h"
57 
58 typedef struct SortSupportData *SortSupport;
59 
60 typedef struct SortSupportData
61 {
62  /*
63  * These fields are initialized before calling the BTSORTSUPPORT function
64  * and should not be changed later.
65  */
66  MemoryContext ssup_cxt; /* Context containing sort info */
67  Oid ssup_collation; /* Collation to use, or InvalidOid */
68 
69  /*
70  * Additional sorting parameters; but unlike ssup_collation, these can be
71  * changed after BTSORTSUPPORT is called, so don't use them in selecting
72  * sort support functions.
73  */
74  bool ssup_reverse; /* descending-order sort? */
75  bool ssup_nulls_first; /* sort nulls first? */
76 
77  /*
78  * These fields are workspace for callers, and should not be touched by
79  * opclass-specific functions.
80  */
81  AttrNumber ssup_attno; /* column number to sort */
82 
83  /*
84  * ssup_extra is zeroed before calling the BTSORTSUPPORT function, and is
85  * not touched subsequently by callers.
86  */
87  void *ssup_extra; /* Workspace for opclass functions */
88 
89  /*
90  * Function pointers are zeroed before calling the BTSORTSUPPORT function,
91  * and must be set by it for any acceleration methods it wants to supply.
92  * The comparator pointer must be set, others are optional.
93  */
94 
95  /*
96  * Comparator function has the same API as the traditional btree
97  * comparison function, ie, return <0, 0, or >0 according as x is less
98  * than, equal to, or greater than y. Note that x and y are guaranteed
99  * not null, and there is no way to return null either. Do not return
100  * INT_MIN, as callers are allowed to negate the result before using it.
101  *
102  * This may be either the authoritative comparator, or the abbreviated
103  * comparator. Core code may switch this over the initial preference of
104  * an opclass support function despite originally indicating abbreviation
105  * was applicable, by assigning the authoritative comparator back.
106  */
107  int (*comparator) (Datum x, Datum y, SortSupport ssup);
108 
109  /*
110  * "Abbreviated key" infrastructure follows.
111  *
112  * All callbacks must be set by sortsupport opclasses that make use of
113  * this optional additional infrastructure (unless for whatever reasons
114  * the opclass doesn't proceed with abbreviation, in which case
115  * abbrev_converter must not be set).
116  *
117  * This allows opclass authors to supply a conversion routine, used to
118  * create an alternative representation of the underlying type (an
119  * "abbreviated key"). This representation must be pass-by-value and
120  * typically will use some ad-hoc format that only the opclass has
121  * knowledge of. An alternative comparator, used only with this
122  * alternative representation must also be provided (which is assigned to
123  * "comparator"). This representation is a simple approximation of the
124  * original Datum. It must be possible to compare datums of this
125  * representation with each other using the supplied alternative
126  * comparator, and have any non-zero return value be a reliable proxy for
127  * what a proper comparison would indicate. Returning zero from the
128  * alternative comparator does not indicate equality, as with a
129  * conventional support routine 1, though -- it indicates that it wasn't
130  * possible to determine how the two abbreviated values compared. A
131  * proper comparison, using "abbrev_full_comparator"/
132  * ApplySortAbbrevFullComparator() is therefore required. In many cases
133  * this results in most or all comparisons only using the cheap
134  * alternative comparison func, which is typically implemented as code
135  * that compiles to just a few CPU instructions. CPU cache miss penalties
136  * are expensive; to get good overall performance, sort infrastructure
137  * must heavily weigh cache performance.
138  *
139  * Opclass authors must consider the final cardinality of abbreviated keys
140  * when devising an encoding scheme. It's possible for a strategy to work
141  * better than an alternative strategy with one usage pattern, while the
142  * reverse might be true for another usage pattern. All of these factors
143  * must be considered.
144  */
145 
146  /*
147  * "abbreviate" concerns whether or not the abbreviated key optimization
148  * is applicable in principle (that is, the sortsupport routine needs to
149  * know if its dealing with a key where an abbreviated representation can
150  * usefully be packed together. Conventionally, this is the leading
151  * attribute key). Note, however, that in order to determine that
152  * abbreviation is not in play, the core code always checks whether or not
153  * the opclass has set abbrev_converter. This is a one way, one time
154  * message to the opclass.
155  */
157 
158  /*
159  * Converter to abbreviated format, from original representation. Core
160  * code uses this callback to convert from a pass-by-reference "original"
161  * Datum to a pass-by-value abbreviated key Datum. Note that original is
162  * guaranteed NOT NULL, because it doesn't make sense to factor NULLness
163  * into ad-hoc cost model.
164  *
165  * abbrev_converter is tested to see if abbreviation is in play. Core
166  * code may set it to NULL to indicate abbreviation should not be used
167  * (which is something sortsupport routines need not concern themselves
168  * with). However, sortsupport routines must not set it when it is
169  * immediately established that abbreviation should not proceed (e.g., for
170  * !abbreviate calls, or due to platform-specific impediments to using
171  * abbreviation).
172  */
173  Datum (*abbrev_converter) (Datum original, SortSupport ssup);
174 
175  /*
176  * abbrev_abort callback allows clients to verify that the current
177  * strategy is working out, using a sortsupport routine defined ad-hoc
178  * cost model. If there is a lot of duplicate abbreviated keys in
179  * practice, it's useful to be able to abandon the strategy before paying
180  * too high a cost in conversion (perhaps certain opclass-specific
181  * adaptations are useful too).
182  */
183  bool (*abbrev_abort) (int memtupcount, SortSupport ssup);
184 
185  /*
186  * Full, authoritative comparator for key that an abbreviated
187  * representation was generated for, used when an abbreviated comparison
188  * was inconclusive (by calling ApplySortComparatorFull()), or used to
189  * replace "comparator" when core system ultimately decides against
190  * abbreviation.
191  */
192  int (*abbrev_full_comparator) (Datum x, Datum y, SortSupport ssup);
194 
195 
196 /*
197  * Apply a sort comparator function and return a 3-way comparison result.
198  * This takes care of handling reverse-sort and NULLs-ordering properly.
199  */
200 static inline int
201 ApplySortComparator(Datum datum1, bool isNull1,
202  Datum datum2, bool isNull2,
203  SortSupport ssup)
204 {
205  int compare;
206 
207  if (isNull1)
208  {
209  if (isNull2)
210  compare = 0; /* NULL "=" NULL */
211  else if (ssup->ssup_nulls_first)
212  compare = -1; /* NULL "<" NOT_NULL */
213  else
214  compare = 1; /* NULL ">" NOT_NULL */
215  }
216  else if (isNull2)
217  {
218  if (ssup->ssup_nulls_first)
219  compare = 1; /* NOT_NULL ">" NULL */
220  else
221  compare = -1; /* NOT_NULL "<" NULL */
222  }
223  else
224  {
225  compare = ssup->comparator(datum1, datum2, ssup);
226  if (ssup->ssup_reverse)
227  compare = -compare;
228  }
229 
230  return compare;
231 }
232 
233 /*
234  * Apply a sort comparator function and return a 3-way comparison using full,
235  * authoritative comparator. This takes care of handling reverse-sort and
236  * NULLs-ordering properly.
237  */
238 static inline int
240  Datum datum2, bool isNull2,
241  SortSupport ssup)
242 {
243  int compare;
244 
245  if (isNull1)
246  {
247  if (isNull2)
248  compare = 0; /* NULL "=" NULL */
249  else if (ssup->ssup_nulls_first)
250  compare = -1; /* NULL "<" NOT_NULL */
251  else
252  compare = 1; /* NULL ">" NOT_NULL */
253  }
254  else if (isNull2)
255  {
256  if (ssup->ssup_nulls_first)
257  compare = 1; /* NOT_NULL ">" NULL */
258  else
259  compare = -1; /* NOT_NULL "<" NULL */
260  }
261  else
262  {
263  compare = ssup->abbrev_full_comparator(datum1, datum2, ssup);
264  if (ssup->ssup_reverse)
265  compare = -compare;
266  }
267 
268  return compare;
269 }
270 
271 /* Other functions in utils/sort/sortsupport.c */
272 extern void PrepareSortSupportComparisonShim(Oid cmpFunc, SortSupport ssup);
273 extern void PrepareSortSupportFromOrderingOp(Oid orderingOp, SortSupport ssup);
274 extern void PrepareSortSupportFromIndexRel(Relation indexRel, int16 strategy,
275  SortSupport ssup);
276 
277 #endif /* SORTSUPPORT_H */
struct SortSupportData * SortSupport
Definition: sortsupport.h:58
signed short int16
Definition: c.h:283
bool ssup_nulls_first
Definition: sortsupport.h:75
void PrepareSortSupportFromIndexRel(Relation indexRel, int16 strategy, SortSupport ssup)
Definition: sortsupport.c:160
void PrepareSortSupportComparisonShim(Oid cmpFunc, SortSupport ssup)
Definition: sortsupport.c:67
unsigned int Oid
Definition: postgres_ext.h:31
char bool
Definition: c.h:247
static int compare(const void *arg1, const void *arg2)
Definition: geqo_pool.c:145
MemoryContext ssup_cxt
Definition: sortsupport.h:66
int(* comparator)(Datum x, Datum y, SortSupport ssup)
Definition: sortsupport.h:107
int(* abbrev_full_comparator)(Datum x, Datum y, SortSupport ssup)
Definition: sortsupport.h:192
Datum(* abbrev_converter)(Datum original, SortSupport ssup)
Definition: sortsupport.h:173
void * ssup_extra
Definition: sortsupport.h:87
struct SortSupportData SortSupportData
void PrepareSortSupportFromOrderingOp(Oid orderingOp, SortSupport ssup)
Definition: sortsupport.c:133
uintptr_t Datum
Definition: postgres.h:372
AttrNumber ssup_attno
Definition: sortsupport.h:81
bool(* abbrev_abort)(int memtupcount, SortSupport ssup)
Definition: sortsupport.h:183
static int ApplySortComparator(Datum datum1, bool isNull1, Datum datum2, bool isNull2, SortSupport ssup)
Definition: sortsupport.h:201
int16 AttrNumber
Definition: attnum.h:21
static int ApplySortAbbrevFullComparator(Datum datum1, bool isNull1, Datum datum2, bool isNull2, SortSupport ssup)
Definition: sortsupport.h:239