Opengauss kernel analysis - statistics and row count estimation

Catalog

Statistics

Row count estimation

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SQL The engine performs query mainly through lexical and grammatical parsing 、 Query rewriting 、 Query Planning and plan execution . among , In the process of Query Planning , To generate an executable optimal plan , First, generate the path , And because of the diversity of paths , Therefore, the path needs to be eliminated . At present, the path selection of the optimizer is mainly based on the estimated cost , So this kind of optimizer is also called cost based optimizer （Cost Based Optimization, CBO）. Relative to logical optimization , This optimization method is physical optimization ： According to the distribution of data （ Statistics ） To evaluate the query execution path , Select a path with the least execution cost from the optional paths to execute , For example, whether to select an index SeqScan vs. IndexScan, Choose which index , What kind of connection order is selected for the association of two tables , What specific algorithm to choose .

When estimating the cost , The number of rows that need to use the base table or the join table , And a lot of the time , The optimizer cannot get the exact row value , Therefore, the number of rows needs to be estimated （Cardinality Estimation）, Then calculate the cost .

Statistics

Statistics are the basis for physical optimization , Statistics from table information . The characteristics describing the base table data include unique values 、MCV（Most Common Value） It's worth waiting for , For line count estimation .

Table-Level Table level statistics , Stored in the system table pg_class.

relptuples Total tuples ： Describes the number of tuples corresponding to the table .

relpages Total pages ： The number of disk pages corresponding to the description table .

Column-Level Column level statistics , Stored in the system table pg_statistics, You can also use views pg_stats View the data .

Starelid： Tabular oid.

Staattnum： Table attribute number .

stadistinct： It is used to describe the only non - in the field NULL Number of data values , It is generally used to estimate the size of a set after grouping ,Join Result set size .

stanullfrac： Used to describe... In the current column NULL The percentage of value in the total .

Attribute group {stakind1, stanumbers1, stavalues1} constitute PG_STATISTIC A card slot in the watch , stay PG_STATISTIC Table has 5 Slots . In general , The first card slot stores MCV（Most Common Value） Information ： Describes a set of values that occur more frequently than a certain percentage , Sort according to the frequency of occurrence , Usually used to indicate which values are skewed . The second card slot stores Histogram Histogram information , Describe except NULL value 、MCV The distribution of values other than , It is generally used to estimate the selection rate .

With MCV Card slot as an example attribute “stakind1” The type of identification card slot is MCV, among “1” by “STATISTIC_KIND_MCV” The enumerated values ; attribute stanumbers1 And properties stavalues1 Record MCV Specific content of , among stavalues1 Record key value ,stanumbers1 Record key Corresponding frequency .

The system tables pg_statistics The definition of is in the file pg_statistic.h in .

#define STATISTIC_KIND_MCV    1#define STATISTIC_KIND_HISTOGRAM  2#define STATISTIC_KIND_CORRELATION    3#define STATISTIC_KIND_MCELEM  4#define STATISTIC_KIND_DECHIST    5

Statistics are provided through analyze Command to get .

surface tt Of oid by 40960, Yes 10000 Row data occupied 345 individual pages page . The first 1 Column unique1 The distribution of can be obtained from histogram information , Histogram has 100 Intervals , And there are no null values and MCV. The first 16 Column string4 The distribution of can be determined by MCV information acquisition , This column has 4 individual distinct value ”AAAAxx” ,”HHHHxx” , “OOOOxx” , “VVVVxx” ,4 The distribution frequency of each value has 0.25.

Row count estimation

Line count estimation is the basis of cost estimation , Extrapolation from base table statistics , Estimate base table baserel、Join Intermediate result set joinrel、Aggregation Result set size in , Prepare for cost estimation .

SQL Queries often have where constraint （ Filter conditions ）, such as SELECT * FROM tt WHERE string4 = 'AAAAxx'. Knowing the selection rate of constraints , That is, we know the proportion of the results to be scanned through the scanning path or the proportion of tuples obtained through the connection operation , From this ratio, we can calculate the number of intermediate results and final results , These quantities are then used to calculate the cost .

Here we focus on the simple query of the base table —— be based on OpExpr Type selection rate calculation , The processing function is in the clause_selectivity. If it is a filter condition, call restriction_selectivity Function to get OpExpr Selection rate of expression , If it is a connection condition, call join_selectivity Function to get the selection rate .

SELECT * FROM tt WHERE string4 = 'AAAAxx' For filter conditions , call restriction_selectivity Estimate the selection rate .

restriction_selectivity The function recognizes string4 = 'AAAAxx' Is shaped like Var = Const Equivalence constraint , The constraint selective evaluation function of the operator is stored in the system table PG_OPERATOR,opno = 93 The corresponding selection rate calculation function is eqsel, adopt eqsel Function call var_eq_const Function to estimate the selection rate . In the process ,var_eq_const The function reads PG_STATISTIC In the table string4 Column distribution information , And make use of MCV The selection rate for direct return of information is 0.25.

  function set_baserel_size_estimates Calculate the estimated number of rows .

Function call relationship ：standard_planner-> subquery_planner-> grouping_planner-> query_planner-> make_one_rel-> set_base_rel_sizes-> set_rel_size-> set_plain_rel_size-> set_baserel_size_estimates-> clauselist_selectivity-> clause_selectivity-> restriction_selectivity-> OidFunctionCall4Coll-> eqsel->var_eq_const