PolarDB-PG原理解读——ANALYZE 源码解读（二）

PolarDB PostgreSQL版（以下简称 PolarDB-PG）是一款阿里云自主研发的企业级数据库产品，采用计算存储分离架构，兼容 PostgreSQL 与 Oracle。PolarDB-PG 的存储与计算能力均可横向扩展，具有高可靠、高可用、弹性扩展等企业级数据库特性。同时，PolarDB-PG 具有大规模并行计算能力，可以应对 OLTP 与 OLAP 混合负载；还具有时空、向量、搜索、图谱等多模创新特性，可以满足企业对数据处理日新月异的新需求。

Most Common Values (MCV)

/*
 * In a "most common values" slot, staop is the OID of the "=" operator
 * used to decide whether values are the same or not, and stacoll is the
 * collation used (same as column's collation).  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
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对于一个列中的 最常见值，在 staop 中保存 = 运算符来决定一个值是否等于一个最常见值。在 stavalues 中保存了该列中最常见的 K 个非空值，stanumbers 中分别保存了这 K 个值出现的频率。

Histogram

/*
 * A "histogram" slot describes the distribution of scalar data.  staop is
 * the OID of the "<" operator that describes the sort ordering, and stacoll
 * is the relevant collation.  (In theory more than one histogram could appear,
 * if a datatype has more than one useful sort operator or we care about more
 * than one collation.  Currently the collation will always be that of the
 * underlying column.)  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
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表示一个（数值）列的数据分布直方图。staop 保存 < 运算符用于决定数据分布的排序顺序。stavalues 包含了能够将该列的非空值划分到 M - 1 个容量接近的桶中的 M 个非空值。如果该列中已经有了 MCV 的槽，那么数据分布直方图中将不包含 MCV 中的值，以获得更精确的数据分布。

Correlation

/*
 * 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 with the collation identified by
 * stacoll.  (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
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在 stanumbers 中保存数据值和它们的实际元组位置的相关系数。