规避途径一: 通过 COUNT 子查询 使用下面的方式, 查询时间能缩短一半以上 SELECT COUNT(col) FROM( SELECTDISTINCTfield_1AScolFROMtable_1 ) TEMP 规避途径二: 通过 COUNT_DISTINCT 扩展 针对这个性能问题的扩展count_distinct, 安装之后可以使用COUNT_DISTINCT()函数代替COUNT(DISTINCT ...), 但是缺点是费内存, ...
In PostgreSQL, you can use the DISTINCT clause to retrieve unique rows based on one or more columns. By specifying multiple columns with DISTINCT, you filter the results to return only the unique combinations of the specified columns. Additionally, PostgreSQL provides a DISTINCT ON clause that all...
} constval;/* for EEOP_FUNCEXPR_* / NULLIF / DISTINCT *///对于EEOP_FUNCEXPR_* / NULLIF / DISTINCTstruct{//函数的检索数据FmgrInfo *finfo;/* function's lookup data *///参数信息等FunctionCallInfo fcinfo_data;/* arguments etc *//* faster to access without additional indirection: *///无...
Sometimes multicolumn indexes are best, but sometimes it's better to create separate indexes and rely on the index-combination feature.Forexample,ifyour workload includes a mixofqueries that sometimes involve only column x, sometimes only column y,andsometimes both columns, you might choosetocreate...
如果列数据类型仅仅支持等于运算符,可以使用compute_distinct_stats函数进行唯一值的分析。 如果列数据类型不支持上述运算,那么只能使用compute_trivial_stats进行分析了。 这里我们以数值scalar类型的列为例,学习一下compute_scalar_stats函数计算统计数据。当我们可以找到数据类型的“=”和“<”运算符时,我们使用compute_...
1. Distinctness in Multiple Columns When you use DISTINCT with multiple columns, the clause considers each combination of column values as a unique row. For instance, (city, country) will only consider rows unique if both values in the pair are unique together. ...
Distinct Elements Count Histogram Length Histogram Bounds Histogram ANALYZE 在 PostgreSQL 内核中的执行流程 compute_trivial_stats compute_distinct_stats compute_scalar_stats 总结 References 背景 PostgreSQL 在优化器中为一个查询树输出一个执行效率最高的物理计划树。其中,执行效率高低的衡量是通过代价估算实现的。
PostgreSQL – Select Distinct on Multiple Columns PostgreSQL – Select Count PostgreSQL – UPDATE Table PostgreSQL – DELETE Rows from Table PostgreSQL – Add a Column to Table PostgreSQL – Delete a Column from Table PostgreSQL – Make a Column PRIMARY KEY ...
For each group, it counts the number of rows using the COUNT() function. 5) Using PostgreSQL GROUP BY with multiple columns The following example uses a GROUP BY clause to group rows by values in two columns: SELECT customer_id, staff_id, SUM(amount) FROM payment GROUP BY staff_id, ...
distinct from w.locktype and r.database is not distinct from w.database and r.relation is not distinct from w.relation and r.page is not distinct from w.page and r.tuple is not distinct from w.tuple and r.classid is not distinct from w.classid and r.objid is not distinct from ...