华为openGauss数据库源码解析——B+树操作(1)

首先介绍一下索引的整体结构，再具体介绍一下其中B+树索引的一些特性

索引结构

数据存储：索引数据页头和与普通数据表页头一样的结构，占用24个字节，ItemIds占用4个字节；

每个索引entries结构为IndexTupleData+Bitmap+Value，其中IndexTupleData占8个字节，Bitmap占4个字节，Value占4字节，合计占用16个字节，数据结构如下：

    typedef struct IndexTupleData {
        ItemPointerData t_tid; /* reference TID to heap tuple */

        /* ---------------
         * t_info is laid out in the following fashion:
         *
         * 15th (high) bit: has nulls
         * 14th bit: has var-width attributes
         * 13th bit: AM-defined meaning
         * 12-0 bit: size of tuple
         * ---------------
         */

        unsigned short t_info; /* various info about tuple */

    } IndexTupleData; /* MORE DATA FOLLOWS AT END OF STRUCT */

    typedef IndexTupleData* IndexTuple;

    typedef struct IndexAttributeBitMapData {