diff options
Diffstat (limited to 'src/include/catalog/pg_statistic.h')
| -rw-r--r-- | src/include/catalog/pg_statistic.h | 177 |
1 files changed, 146 insertions, 31 deletions
diff --git a/src/include/catalog/pg_statistic.h b/src/include/catalog/pg_statistic.h index 2f39bea3245..8d6a6b37c16 100644 --- a/src/include/catalog/pg_statistic.h +++ b/src/include/catalog/pg_statistic.h @@ -8,7 +8,7 @@ * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * - * $Id: pg_statistic.h,v 1.10 2001/01/24 19:43:22 momjian Exp $ + * $Id: pg_statistic.h,v 1.11 2001/05/07 00:43:25 tgl Exp $ * * NOTES * the genbki.sh script reads this file and generates .bki @@ -36,40 +36,91 @@ CATALOG(pg_statistic) /* These fields form the unique key for the entry: */ Oid starelid; /* relation containing attribute */ int2 staattnum; /* attribute (column) stats are for */ - Oid staop; /* '<' comparison op used for lo/hi vals */ + + /* the fraction of the column's entries that are NULL: */ + float4 stanullfrac; /* - * Note: the current VACUUM code will never produce more than one - * entry per column, but in theory there could be multiple entries if - * a datatype has more than one useful ordering operator. Also, the - * current code will not write an entry unless it found at least one - * non-NULL value in the column; so the remaining fields will never be - * NULL. + * stawidth is the average width in bytes of non-null entries. For + * fixed-width datatypes this is of course the same as the typlen, but + * for varlena types it is more useful. Note that this is the average + * width of the data as actually stored, post-TOASTing (eg, for a + * moved-out-of-line value, only the size of the pointer object is + * counted). This is the appropriate definition for the primary use of + * the statistic, which is to estimate sizes of in-memory hash tables of + * tuples. + */ + int4 stawidth; + + /* ---------------- + * stadistinct indicates the (approximate) number of distinct non-null + * data values in the column. The interpretation is: + * 0 unknown or not computed + * > 0 actual number of distinct values + * < 0 negative of multiplier for number of rows + * The special negative case allows us to cope with columns that are + * unique (stadistinct = -1) or nearly so (for example, a column in + * which values appear about twice on the average could be represented + * by stadistinct = -0.5). Because the number-of-rows statistic in + * pg_class may be updated more frequently than pg_statistic is, it's + * important to be able to describe such situations as a multiple of + * the number of rows, rather than a fixed number of distinct values. + * But in other cases a fixed number is correct (eg, a boolean column). + * ---------------- + */ + float4 stadistinct; + + /* ---------------- + * To allow keeping statistics on different kinds of datatypes, + * we do not hard-wire any particular meaning for the remaining + * statistical fields. Instead, we provide several "slots" in which + * statistical data can be placed. Each slot includes: + * kind integer code identifying kind of data + * op OID of associated operator, if needed + * numbers float4 array (for statistical values) + * values text array (for representations of data values) + * The ID and operator fields are never NULL; they are zeroes in an + * unused slot. The numbers and values fields are NULL in an unused + * slot, and might also be NULL in a used slot if the slot kind has + * no need for one or the other. + * ---------------- */ + int2 stakind1; + int2 stakind2; + int2 stakind3; + int2 stakind4; + + Oid staop1; + Oid staop2; + Oid staop3; + Oid staop4; + /* - * These fields contain the stats about the column indicated by the - * key + * THE REST OF THESE ARE VARIABLE LENGTH FIELDS, and may even be absent + * (NULL). They cannot be accessed as C struct entries; you have to use + * the full field access machinery (heap_getattr) for them. We declare + * them here for the catalog machinery. */ - float4 stanullfrac; /* the fraction of the entries that are - * NULL */ - float4 stacommonfrac; /* the fraction that are the most common - * val */ + + float4 stanumbers1[1]; + float4 stanumbers2[1]; + float4 stanumbers3[1]; + float4 stanumbers4[1]; /* - * THE REST OF THESE ARE VARIABLE LENGTH FIELDS. They cannot be - * accessed as C struct entries; you have to use the full field access - * machinery (heap_getattr) for them. - * - * All three of these are text representations of data values of the - * column's data type. To re-create the actual Datum, do - * datatypein(textout(givenvalue)). + * Values in these text arrays are external representations of values + * of the column's data type. To re-create the actual Datum, do + * datatypein(textout(arrayelement)). */ - text stacommonval; /* most common non-null value in column */ - text staloval; /* smallest non-null value in column */ - text stahival; /* largest non-null value in column */ + text stavalues1[1]; + text stavalues2[1]; + text stavalues3[1]; + text stavalues4[1]; } FormData_pg_statistic; +#define STATISTIC_NUM_SLOTS 4 + /* ---------------- * Form_pg_statistic corresponds to a pointer to a tuple with * the format of pg_statistic relation. @@ -81,14 +132,78 @@ typedef FormData_pg_statistic *Form_pg_statistic; * compiler constants for pg_statistic * ---------------- */ -#define Natts_pg_statistic 8 +#define Natts_pg_statistic 21 #define Anum_pg_statistic_starelid 1 #define Anum_pg_statistic_staattnum 2 -#define Anum_pg_statistic_staop 3 -#define Anum_pg_statistic_stanullfrac 4 -#define Anum_pg_statistic_stacommonfrac 5 -#define Anum_pg_statistic_stacommonval 6 -#define Anum_pg_statistic_staloval 7 -#define Anum_pg_statistic_stahival 8 +#define Anum_pg_statistic_stanullfrac 3 +#define Anum_pg_statistic_stawidth 4 +#define Anum_pg_statistic_stadistinct 5 +#define Anum_pg_statistic_stakind1 6 +#define Anum_pg_statistic_stakind2 7 +#define Anum_pg_statistic_stakind3 8 +#define Anum_pg_statistic_stakind4 9 +#define Anum_pg_statistic_staop1 10 +#define Anum_pg_statistic_staop2 11 +#define Anum_pg_statistic_staop3 12 +#define Anum_pg_statistic_staop4 13 +#define Anum_pg_statistic_stanumbers1 14 +#define Anum_pg_statistic_stanumbers2 15 +#define Anum_pg_statistic_stanumbers3 16 +#define Anum_pg_statistic_stanumbers4 17 +#define Anum_pg_statistic_stavalues1 18 +#define Anum_pg_statistic_stavalues2 19 +#define Anum_pg_statistic_stavalues3 20 +#define Anum_pg_statistic_stavalues4 21 + +/* + * Currently, three statistical slot "kinds" are defined: most common values, + * histogram, and correlation. Additional "kinds" will probably appear in + * future to help cope with non-scalar datatypes. + * + * Code reading the pg_statistic relation should not assume that a particular + * data "kind" will appear in any particular slot. Instead, search the + * stakind fields to see if the desired data is available. + */ + +/* + * In a "most common values" slot, staop is the OID of the "=" operator + * used to decide whether values are the same or not. stavalues contains + * the K most common non-null values appearing in the column, and stanumbers + * contains their frequencies (fractions of total row count). The values + * shall be ordered in decreasing frequency. Note that since the arrays are + * variable-size, K may be chosen by the statistics collector. Values should + * not appear in MCV unless they have been observed to occur more than once; + * a unique column will have no MCV slot. + */ +#define STATISTIC_KIND_MCV 1 + +/* + * A "histogram" slot describes the distribution of scalar data. staop is + * the OID of the "<" operator that describes the sort ordering. (In theory, + * more than one histogram could appear, if a datatype has more than one + * useful sort operator.) stavalues contains M (>=2) non-null values that + * divide the non-null column data values into M-1 bins of approximately equal + * population. The first stavalues item is the MIN and the last is the MAX. + * stanumbers is not used and should be NULL. IMPORTANT POINT: if an MCV + * slot is also provided, then the histogram describes the data distribution + * *after removing the values listed in MCV* (thus, it's a "compressed + * histogram" in the technical parlance). This allows a more accurate + * representation of the distribution of a column with some very-common + * values. In a column with only a few distinct values, it's possible that + * the MCV list describes the entire data population; in this case the + * histogram reduces to empty and should be omitted. + */ +#define STATISTIC_KIND_HISTOGRAM 2 + +/* + * A "correlation" slot describes the correlation between the physical order + * of table tuples and the ordering of data values of this column, as seen + * by the "<" operator identified by staop. (As with the histogram, more + * than one entry could theoretically appear.) stavalues is not used and + * should be NULL. stanumbers contains a single entry, the correlation + * coefficient between the sequence of data values and the sequence of + * their actual tuple positions. The coefficient ranges from +1 to -1. + */ +#define STATISTIC_KIND_CORRELATION 3 #endif /* PG_STATISTIC_H */ |