PostgreSQL中的BlockIdData解释

2020-06-17 00:00:00 专区是一个字节顺序都会

本文是基于当前的PostgreSQL master分支进行介绍的，当前commitid为087d3d0583cf292146a7385746d1f5b53eeeaee6。

只要学习过PostgreSQL存储结构的同学，对于ctid一定不会陌生。ctid是PostgreSQL是对于物理tuple的标识符，不同对象存在相同的ctid。

ctid由两部分构成，page顺序值以及tuple顺序值，一般表示为，'(12, 33)'。page顺序值指的是page在所有segment中的顺序，是一个且连续的数值；tuple顺序值指的是在当前page页面中的顺序值。这两项就能够标识tuple了。

/*
 * ItemPointer:
 *
 * This is a pointer to an item within a disk page of a known file
 * (for example, a cross-link from an index to its parent table).
 * ip_blkid tells us which block, ip_posid tells us which entry in
 * the linp (ItemIdData) array we want.
 *
 * Note: because there is an item pointer in each tuple header and index
 * tuple header on disk, it's very important not to waste space with
 * structure padding bytes.  The struct is designed to be six bytes long
 * (it contains three int16 fields) but a few compilers will pad it to
 * eight bytes unless coerced.  We apply appropriate persuasion where
 * possible.  If your compiler can't be made to play along, you'll waste
 * lots of space.
 */
typedef struct ItemPointerData
{
    BlockIdData ip_blkid;
    OffsetNumber ip_posid;
}

相关文章