join

<!--查询一张表下有哪些索引，如查询report.common_count下所有索引--> select A.SCHEMANAME, A.TABLENAME, A.INDEXNAME, A.TABLESPACE, A.INDEXDEF, B.AMNAME, C.INDEXRELID, C.INDNATTS, C.INDISUNIQUE, C.INDISPRIMARY, C.INDISCLUSTERED, D.DESCRIPTION from PG_AM B left join PG_CLASS F on B.OID = F.RELAM left join PG_STAT_ALL_INDEXES E on F.OID = E.INDEXRELID left join PG_INDEX C on E.INDEXRELID = C.INDEXRELID left outer join PG_DESCRIPTION D on C.INDEXRELID = D.OBJOID, PG_INDEXES A where A.SCHEMANAME = E.SCHEMANAME and A.TABLENAME = E.RELNAME and A.INDEXNAME = E.INDEXRELNAME and E.SCHEMANAME = ' schema_name ' and E.RELNAME =

CONDITIONAL FUNCTIONS IN HIVE Hive supports three types of conditional functions. These functions are listed below: IF( Test Condition, True Value, False Value ) The IF condition evaluates the “Test Condition” and if the “Test Condition” is true, then it returns the “True Value”. Otherwise, it returns the False Value. Example: IF(1=1, 'working', 'not working') returns 'working' COALESCE( value1,value2,... ) The COALESCE function returns the fist not NULL value from the list of values. If all the values in the list are NULL, then it returns NULL. Example: COALESCE(NULL,NULL,5,NULL,4) returns 5

问题： LEFT JOIN 和 LEFT OUTER JOIN 什么区别？ 解决方案： 参考一： https://stackoom.com/question/1hh8/SQL-Server中的LEFT-JOIN与LEFT-OUTER-JOIN 参考二： https://oldbug.net/q/1hh8/LEFT-JOIN-vs-LEFT-OUTER-JOIN-in-SQL-Server 来源： oschina 链接： https://my.oschina.net/stackoom/blog/3217759

当两个表需要join时，如果一个是大表，一个是小表，正常的map-reduce流程需要shuffle，这会导致大表数据在节点间网络传输，常见的优化方式是将小表读到内存中并广播到大表处理，避免shuffle+reduce； 在hive中叫mapjoin（map-side join），配置为 hive.auto.convert.join 在spark中叫BroadcastHashJoin （broadcast hash join） Spark SQL uses broadcast join (aka broadcast hash join) instead of hash join to optimize join queries when the size of one side data is below spark.sql.autoBroadcastJoinThreshold . Broadcast join can be very efficient for joins between a large table (fact) with relatively small tables (dimensions) that could then be used to perform a star-schema join. It can avoid sending all data

对象转换成字符串需要使用toString()方法。 1 var a = function(){ 2 console.log(111); 3 }; 4 var b = a.toString(); 5 console.log(a); 函数式的function 6 console.log(b); 字符串function 7 console.log(typeof a); >>function 8 console.log(typeof b); >>string 数组转换字符串 1.0 1 var a = [1,2,3]; 2 var b = a.toString(); 3 console.log(a); >>[1,2,3] 4 console.log(b); >>"1,2,3" 当我们进行与原始值比较的时候，它们会自动调用toString方法。（+,-,!=,==都会进行转换。） 数组转换字符串 2.0 1 // var a = [1,2,3]; 2 // console.log(a.join()); >>1,2,3 toString()与join()的比较 1 // var a = [1,2,3]; 2 // console.log(a.join()); >>1,2,3 3 // console.log(a.toString()); >>1,2,3 4 // console.log(a

3 月，跳不动了？>>> 在laravel中使用leftJoin添加多个条件时，如select a.* from a left join b on a.id = b.pid and b.status = 1这种类似sql，发现框架自身封装的leftJoin不支持多个参数传递（当然可用写原生sql），laravel框架自身封装的leftJoin方法如下： public function leftJoin($table, $first, $operator = null, $second = null) { return $this->join($table, $first, $operator, $second, 'left'); } 浏览下 \vendor\laravel\framework\src\Illuminate\Database\Query\Builder.php文件，发现join方法可用实现自己想要的left join携带多参数。laravel自身的join方法如下： public function join($table, $one, $operator = null, $two = null, $type = 'inner', $where = false) { // If the first "column" of the join is really a

3 月，跳不动了？>>> -> leftjoin ( 'bill_detail as bl_refund' , function ( $joins ) { $joins -> on ( 'bl_refund.bill_id' , '=' , 'bl.bill_id' ) ; $joins -> where ( 'bl_refund.settle_type' , '=' , 7 ) ; }) 来源： oschina 链接： https://my.oschina.net/guozhouyuan/blog/3211854