FIRST, NULLS FIRST是ORDER BY DESC的默认情况)。 我们可以在创建B-tree索引时通过ASC、DESC、NULLS FIRST和NULLS LAST选项来改变索引的 排序,例如: CREATE INDEX test2_info_nulls_low ON test2 (info NULLS FIRST); CREATE INDEX test3_desc_index ON test3 (id DESC NULLS LAST); 一个以升序存储且将空值...
这意味着对列x上的索引的正向扫描产生满足ORDER BY x(或更详细地,ORDER BY x ASC NULLS LAST)的输出。索引也可以向后扫描,产生满足ORDER BY x DESC的输出(或更详细地,ORDER BY x DESC NULLS FIRST,因为NULLS FIRST是ORDER BY DESC的默认值)。 您可以通过在创建索引时包含ASC,DESC,NULLS FIRST和/或NULLS L...
SELECT * FROM places ORDER BY location <-> point '(101,456)' LIMIT 10; 1. 它找到最接近给定目标点的十个位置。这样做的能力又取决于所使用的特定操作符类。在表61-1中,可以以这种方式使用的操作员列在“Ordering Operators”一栏中。 SP-GiST索引(类似GiST索引)提供支持各种搜索的基础架构。 SP-GiST允...
整个page的结构比Oracle的数据块结构简单多了。 typedef struct PageHeaderData{ /* XXX LSN is member of *any* block, not only page-organized ones */ PageXLogRecPtr pd_lsn; /* LSN: next byte after last byte of xlog * record for last change to this page */ uint16 pd_checksum; /* check...
(relation); if (nblocks > 0) targetBlock = nblocks - 1; } loop: while (targetBlock != InvalidBlockNumber) { /* * Read and exclusive-lock the target block, as well as the other * block if one was given, taking suitable care with lock ordering and * the possibility they are the ...
距离函数是为索引元素调用的,它必须计算从表达式(“indexed-field ordering-operator expression”)定义的值到给定元素的距离。对于叶元素,这只是到索引值的距离。对于内部元素,函数必须返回到子叶元素的最小距离。由于遍历所有子行代价很高,因此允许该函数乐观地低估距离,但以降低搜索效率为代价。但是,决不允许函数高估距...
* * We need to be clever about the direction of scan, the search * conditions, and the tree ordering. We find the first item (or, * if backwards scan, the last item) in the tree that satisfies the * qualifications in the scan key. On success exit, the page containing * the ...
(void) get_equality_op_for_ordering_op(sortOperator, &reverse); } /* Add NULLS FIRST/LAST only if it wouldn't be default */ if (nullsFirst && !reverse) { appendStringInfoString(buf, " NULLS FIRST"); } else if (!nullsFirst && reverse) { appendStringInfoString(buf, " NULLS LAST"...
postgres=# select mode() within group (order by info) from test; mode --- test1 (1 row) 如果按INFO来分组的话,取出出现频率最高的info,实际上这个操作是没有任何意义的,返回值就是所有记录的info的唯一值. postgres=# select mode() within group (order by info) from test group by info; mode...
Acl *proacl =NULL;//ACL结构体Relation rel;//关系HeapTuple tup;//tupleHeapTuple oldtup;//原pg_proc tupleboolnulls[Natts_pg_proc];//是否为nullDatum values[Natts_pg_proc];//值boolreplaces[Natts_pg_proc];//是否替换NameData procname;//名称TupleDesc tupDesc;//tuple描述符boolis_update;//...