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pg_statistic.h
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1 /*-------------------------------------------------------------------------
2  *
3  * pg_statistic.h
4  * definition of the "statistics" system catalog (pg_statistic)
5  *
6  *
7  * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
8  * Portions Copyright (c) 1994, Regents of the University of California
9  *
10  * src/include/catalog/pg_statistic.h
11  *
12  * NOTES
13  * The Catalog.pm module reads this file and derives schema
14  * information.
15  *
16  *-------------------------------------------------------------------------
17  */
18 #ifndef PG_STATISTIC_H
19 #define PG_STATISTIC_H
20 
21 #include "catalog/genbki.h"
22 #include "catalog/pg_statistic_d.h"
23 
24 /* ----------------
25  * pg_statistic definition. cpp turns this into
26  * typedef struct FormData_pg_statistic
27  * ----------------
28  */
29 CATALOG(pg_statistic,2619,StatisticRelationId)
30 {
31  /* These fields form the unique key for the entry: */
32  Oid starelid; /* relation containing attribute */
33  int16 staattnum; /* attribute (column) stats are for */
34  bool stainherit; /* true if inheritance children are included */
35 
36  /* the fraction of the column's entries that are NULL: */
37  float4 stanullfrac;
38 
39  /*
40  * stawidth is the average width in bytes of non-null entries. For
41  * fixed-width datatypes this is of course the same as the typlen, but for
42  * var-width types it is more useful. Note that this is the average width
43  * of the data as actually stored, post-TOASTing (eg, for a
44  * moved-out-of-line value, only the size of the pointer object is
45  * counted). This is the appropriate definition for the primary use of
46  * the statistic, which is to estimate sizes of in-memory hash tables of
47  * tuples.
48  */
49  int32 stawidth;
50 
51  /* ----------------
52  * stadistinct indicates the (approximate) number of distinct non-null
53  * data values in the column. The interpretation is:
54  * 0 unknown or not computed
55  * > 0 actual number of distinct values
56  * < 0 negative of multiplier for number of rows
57  * The special negative case allows us to cope with columns that are
58  * unique (stadistinct = -1) or nearly so (for example, a column in which
59  * non-null values appear about twice on the average could be represented
60  * by stadistinct = -0.5 if there are no nulls, or -0.4 if 20% of the
61  * column is nulls). Because the number-of-rows statistic in pg_class may
62  * be updated more frequently than pg_statistic is, it's important to be
63  * able to describe such situations as a multiple of the number of rows,
64  * rather than a fixed number of distinct values. But in other cases a
65  * fixed number is correct (eg, a boolean column).
66  * ----------------
67  */
68  float4 stadistinct;
69 
70  /* ----------------
71  * To allow keeping statistics on different kinds of datatypes,
72  * we do not hard-wire any particular meaning for the remaining
73  * statistical fields. Instead, we provide several "slots" in which
74  * statistical data can be placed. Each slot includes:
75  * kind integer code identifying kind of data (see below)
76  * op OID of associated operator, if needed
77  * coll OID of relevant collation, or 0 if none
78  * numbers float4 array (for statistical values)
79  * values anyarray (for representations of data values)
80  * The ID, operator, and collation fields are never NULL; they are zeroes
81  * in an unused slot. The numbers and values fields are NULL in an
82  * unused slot, and might also be NULL in a used slot if the slot kind
83  * has no need for one or the other.
84  * ----------------
85  */
86 
87  int16 stakind1;
88  int16 stakind2;
89  int16 stakind3;
90  int16 stakind4;
91  int16 stakind5;
92 
93  Oid staop1;
94  Oid staop2;
95  Oid staop3;
96  Oid staop4;
97  Oid staop5;
98 
99  Oid stacoll1;
100  Oid stacoll2;
101  Oid stacoll3;
102  Oid stacoll4;
103  Oid stacoll5;
104 
105 #ifdef CATALOG_VARLEN /* variable-length fields start here */
106  float4 stanumbers1[1];
107  float4 stanumbers2[1];
108  float4 stanumbers3[1];
109  float4 stanumbers4[1];
110  float4 stanumbers5[1];
111 
112  /*
113  * Values in these arrays are values of the column's data type, or of some
114  * related type such as an array element type. We presently have to cheat
115  * quite a bit to allow polymorphic arrays of this kind, but perhaps
116  * someday it'll be a less bogus facility.
117  */
118  anyarray stavalues1;
119  anyarray stavalues2;
120  anyarray stavalues3;
121  anyarray stavalues4;
122  anyarray stavalues5;
123 #endif
125 
126 #define STATISTIC_NUM_SLOTS 5
127 
128 
129 /* ----------------
130  * Form_pg_statistic corresponds to a pointer to a tuple with
131  * the format of pg_statistic relation.
132  * ----------------
133  */
135 
136 #ifdef EXPOSE_TO_CLIENT_CODE
137 
138 /*
139  * Several statistical slot "kinds" are defined by core PostgreSQL, as
140  * documented below. Also, custom data types can define their own "kind"
141  * codes by mutual agreement between a custom typanalyze routine and the
142  * selectivity estimation functions of the type's operators.
143  *
144  * Code reading the pg_statistic relation should not assume that a particular
145  * data "kind" will appear in any particular slot. Instead, search the
146  * stakind fields to see if the desired data is available. (The standard
147  * function get_attstatsslot() may be used for this.)
148  */
149 
150 /*
151  * The present allocation of "kind" codes is:
152  *
153  * 1-99: reserved for assignment by the core PostgreSQL project
154  * (values in this range will be documented in this file)
155  * 100-199: reserved for assignment by the PostGIS project
156  * (values to be documented in PostGIS documentation)
157  * 200-299: reserved for assignment by the ESRI ST_Geometry project
158  * (values to be documented in ESRI ST_Geometry documentation)
159  * 300-9999: reserved for future public assignments
160  *
161  * For private use you may choose a "kind" code at random in the range
162  * 10000-30000. However, for code that is to be widely disseminated it is
163  * better to obtain a publicly defined "kind" code by request from the
164  * PostgreSQL Global Development Group.
165  */
166 
167 /*
168  * In a "most common values" slot, staop is the OID of the "=" operator
169  * used to decide whether values are the same or not, and stacoll is the
170  * collation used (same as column's collation). stavalues contains
171  * the K most common non-null values appearing in the column, and stanumbers
172  * contains their frequencies (fractions of total row count). The values
173  * shall be ordered in decreasing frequency. Note that since the arrays are
174  * variable-size, K may be chosen by the statistics collector. Values should
175  * not appear in MCV unless they have been observed to occur more than once;
176  * a unique column will have no MCV slot.
177  */
178 #define STATISTIC_KIND_MCV 1
179 
180 /*
181  * A "histogram" slot describes the distribution of scalar data. staop is
182  * the OID of the "<" operator that describes the sort ordering, and stacoll
183  * is the relevant collation. (In theory more than one histogram could appear,
184  * if a datatype has more than one useful sort operator or we care about more
185  * than one collation. Currently the collation will always be that of the
186  * underlying column.) stavalues contains M (>=2) non-null values that
187  * divide the non-null column data values into M-1 bins of approximately equal
188  * population. The first stavalues item is the MIN and the last is the MAX.
189  * stanumbers is not used and should be NULL. IMPORTANT POINT: if an MCV
190  * slot is also provided, then the histogram describes the data distribution
191  * *after removing the values listed in MCV* (thus, it's a "compressed
192  * histogram" in the technical parlance). This allows a more accurate
193  * representation of the distribution of a column with some very-common
194  * values. In a column with only a few distinct values, it's possible that
195  * the MCV list describes the entire data population; in this case the
196  * histogram reduces to empty and should be omitted.
197  */
198 #define STATISTIC_KIND_HISTOGRAM 2
199 
200 /*
201  * A "correlation" slot describes the correlation between the physical order
202  * of table tuples and the ordering of data values of this column, as seen
203  * by the "<" operator identified by staop with the collation identified by
204  * stacoll. (As with the histogram, more than one entry could theoretically
205  * appear.) stavalues is not used and should be NULL. stanumbers contains
206  * a single entry, the correlation coefficient between the sequence of data
207  * values and the sequence of their actual tuple positions. The coefficient
208  * ranges from +1 to -1.
209  */
210 #define STATISTIC_KIND_CORRELATION 3
211 
212 /*
213  * A "most common elements" slot is similar to a "most common values" slot,
214  * except that it stores the most common non-null *elements* of the column
215  * values. This is useful when the column datatype is an array or some other
216  * type with identifiable elements (for instance, tsvector). staop contains
217  * the equality operator appropriate to the element type, and stacoll
218  * contains the collation to use with it. stavalues contains
219  * the most common element values, and stanumbers their frequencies. Unlike
220  * MCV slots, frequencies are measured as the fraction of non-null rows the
221  * element value appears in, not the frequency of all rows. Also unlike
222  * MCV slots, the values are sorted into the element type's default order
223  * (to support binary search for a particular value). Since this puts the
224  * minimum and maximum frequencies at unpredictable spots in stanumbers,
225  * there are two extra members of stanumbers, holding copies of the minimum
226  * and maximum frequencies. Optionally, there can be a third extra member,
227  * which holds the frequency of null elements (expressed in the same terms:
228  * the fraction of non-null rows that contain at least one null element). If
229  * this member is omitted, the column is presumed to contain no null elements.
230  *
231  * Note: in current usage for tsvector columns, the stavalues elements are of
232  * type text, even though their representation within tsvector is not
233  * exactly text.
234  */
235 #define STATISTIC_KIND_MCELEM 4
236 
237 /*
238  * A "distinct elements count histogram" slot describes the distribution of
239  * the number of distinct element values present in each row of an array-type
240  * column. Only non-null rows are considered, and only non-null elements.
241  * staop contains the equality operator appropriate to the element type,
242  * and stacoll contains the collation to use with it.
243  * stavalues is not used and should be NULL. The last member of stanumbers is
244  * the average count of distinct element values over all non-null rows. The
245  * preceding M (>=2) members form a histogram that divides the population of
246  * distinct-elements counts into M-1 bins of approximately equal population.
247  * The first of these is the minimum observed count, and the last the maximum.
248  */
249 #define STATISTIC_KIND_DECHIST 5
250 
251 /*
252  * A "length histogram" slot describes the distribution of range lengths in
253  * rows of a range-type column. stanumbers contains a single entry, the
254  * fraction of empty ranges. stavalues is a histogram of non-empty lengths, in
255  * a format similar to STATISTIC_KIND_HISTOGRAM: it contains M (>=2) range
256  * values that divide the column data values into M-1 bins of approximately
257  * equal population. The lengths are stored as float8s, as measured by the
258  * range type's subdiff function. Only non-null rows are considered.
259  */
260 #define STATISTIC_KIND_RANGE_LENGTH_HISTOGRAM 6
261 
262 /*
263  * A "bounds histogram" slot is similar to STATISTIC_KIND_HISTOGRAM, but for
264  * a range-type column. stavalues contains M (>=2) range values that divide
265  * the column data values into M-1 bins of approximately equal population.
266  * Unlike a regular scalar histogram, this is actually two histograms combined
267  * into a single array, with the lower bounds of each value forming a
268  * histogram of lower bounds, and the upper bounds a histogram of upper
269  * bounds. Only non-NULL, non-empty ranges are included.
270  */
271 #define STATISTIC_KIND_BOUNDS_HISTOGRAM 7
272 
273 #endif /* EXPOSE_TO_CLIENT_CODE */
274 
275 #endif /* PG_STATISTIC_H */
signed short int16
Definition: c.h:345
unsigned int Oid
Definition: postgres_ext.h:31
FormData_pg_statistic * Form_pg_statistic
Definition: pg_statistic.h:134
signed int int32
Definition: c.h:346
CATALOG(pg_statistic, 2619, StatisticRelationId)
Definition: pg_statistic.h:29
FormData_pg_statistic
Definition: pg_statistic.h:124
float float4
Definition: c.h:490