postgres 源码解析 45 btree分裂流程_bt_split

在PostgreSQL中，btree分裂操作是在插入新元组时，如果页面已满，需要将页面分裂成两半以便为新元组腾出空间时发生的。\_bt\_split 函数负责执行这个任务。以下是该函数的核心代码片段：

/*
 *    _bt_split() -- split a full page into two
 *
 * The original page is on the left, and the new page is created on the right.
 *
 * The split key is chosen among the page's current entries to keep the split
 * about even.  (This is called "infinity mass splitting" in the TIDBIT paper
 * that described this technique.)  We also ensure that the right sibling will
 * be at least half full, by moving multiple items to the right if necessary.
 */
void
_bt_split(Relation rel,
          Buffer lbuf, Buffer rbuf,
          BTStack stack,
          int newitemoff,
          IndexTuple itup,
          Size itemsize,
          bool split_only)
{
    Page lpage,
        rpage;
    ItemId lpid,
           rpid;
    ItemIdData rpiddata;
    IndexTupleData *itupdata;
    OffsetNumber off;
    OffsetNumber separator;
    OffsetNumber offright;
    OffsetNumber minoff;
    OffsetNumber maxoff;
    bool is_leaf;
    BlockNumber rblkno;
 
    /* XXX cache overflow page */
    lpage = BufferGetPage(lbuf);
    rpage = BufferGetPage(rbuf);
    is_leaf = P_ISLEAF(lpage);
    minoff = P_FIRSTDATAKEY(lpage);
    maxoff = PageGetMaxOffsetNumber(lpage);
 
    /*
     * Select the midpoint in the current page to split.
     *
     * The midpoint algorithm is not quite the "infinity mass" method described
     * in the TIDBIT paper, as we cannot enforce that exactly. The algorithm
     * here is thus:
     *
     *        1. Calculate the "ideal" midpoint, which is the median key value.
     *        2. If the ideal midpoint is the first or last key, or is less than
     *           2 keys from the first or last key, move it forward or backward
     *           until it has the desired number of keys on each side.
     *
     * Note that if there are only two items in the page, the midpoint will be
     * the first or second item, and so by the time we're done moving it, it
     * will have the desired keys on each side.
     */
    separator = (OffsetNumber) (((int) maxoff + minoff) / 2);
    if (separator == minoff || separator == maxoff)
    {
        /*
         * At one end of the page, so move t