PostgreSQL在何处处理 sql查询之三十八-优快云博客

本文详细解析了SQL查询执行过程中如何利用函数指针优化数据访问过程，通过实例展示了如何在不同场景下高效地使用函数指针实现特定的查询逻辑，旨在提升查询性能并简化代码复杂度。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

这里又遇到了函数指针：

executor.h头文件中，定义了 ExecScanAccessMtd 指针，或者定义了一个ExecScanAccessMtd 函数原型的指针

/*
 * prototypes from functions in execScan.c
 */
typedef TupleTableSlot *(*ExecScanAccessMtd) (ScanState *node);

之后，在NodeSeqScan.c处有一个实现：

/* ----------------------------------------------------------------
 *        SeqNext
 *
 *        This is a workhorse for ExecSeqScan
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
SeqNext(SeqScanState *node)
{
    HeapTuple    tuple;
    HeapScanDesc scandesc;
    EState       *estate;
    ScanDirection direction;
    TupleTableSlot *slot;

    /*
     * get information from the estate and scan state
     */
    scandesc = node->ss_currentScanDesc;
    estate = node->ps.state;
    direction = estate->es_direction;
    slot = node->ss_ScanTupleSlot;

    /*
     * get the next tuple from the table
     */
    tuple = heap_getnext(scandesc, direction);

    /*
     * save the tuple and the buffer returned to us by the access methods in
     * our scan tuple slot and return the slot.  Note: we pass 'false' because
     * tuples returned by heap_getnext() are pointers onto disk pages and were
     * not created with palloc() and so should not be pfree()'d.  Note also
     * that ExecStoreTuple will increment the refcount of the buffer; the
     * refcount will not be dropped until the tuple table slot is cleared.
     */
    if (tuple)
        ExecStoreTuple(tuple,    /* tuple to store */
                       slot,    /* slot to store in */
                       scandesc->rs_cbuf,        /* buffer associated with this
                                                 * tuple */
                       false);    /* don't pfree this pointer */
    else
        ExecClearTuple(slot);

    return slot;
}

之后，在下列方法中，把函数指针当作参数进行传递：

TupleTableSlot *
ExecScan(ScanState *node,
         ExecScanAccessMtd accessMtd,    /* function returning a tuple */
         ExecScanRecheckMtd recheckMtd)
{
   ...
}

调用：

TupleTableSlot *
ExecSeqScan(SeqScanState *node)
{
    return ExecScan((ScanState *) node,
                    (ExecScanAccessMtd) SeqNext,
                    (ExecScanRecheckMtd) SeqRecheck);
}