CREATE FUNCTION pg_recovery(regclass, recoveryrow bool DEFAULT true) RETURNS SETOF record

select * from pg_recovery('aa', recoveryrow => false)

typedef struct{    Relation            rel;    -- 当前操作的表    TupleDesc           reltupledesc; -- 表的元信息    TupleConversionMap  *map; -- 表的映射图，即表的数据映射成自定义返回的列    TableScanDesc       scan; -- 扫描表    HTAB                *active_ctid; -- 可见数据的ctid    bool                droppedcolumn; -- 是否删除列} pg_recovery_ctx;

static const struct system_columns_t {    char       *attname;    Oid         atttypid;    int32       atttypmod;    int         attnum;} system_columns[] = {     { "ctid",     TIDOID,  -1, SelfItemPointerAttributeNumber },    { "xmin",     XIDOID,  -1, MinTransactionIdAttributeNumber },    { "cmin",     CIDOID,  -1, MinCommandIdAttributeNumber },    { "xmax",     XIDOID,  -1, MaxTransactionIdAttributeNumber },    { "cmax",     CIDOID,  -1, MaxCommandIdAttributeNumber },    { "tableoid", OIDOID,  -1, TableOidAttributeNumber },    { "recoveryrow",     BOOLOID, -1, DeadFakeAttributeNumber },    { 0 },};

Datumpg_recovery(PG_FUNCTION_ARGS){    FuncCallContext     *funcctx;    pg_recovery_ctx *usr_ctx;        recoveryrow = PG_GETARG_BOOL(1); -- 获取默认参数
    if (SRF_IS_FIRSTCALL()) -- 每条数据，函数都会调用一次，因此需要先初始化数据    {        funcctx = SRF_FIRSTCALL_INIT(); -- 申请上下文        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx); -- 使用内存池
        usr_ctx->rel = heap_open(relid, AccessShareLock); -- 增加读锁        usr_ctx->reltupledesc = RelationGetDescr(usr_ctx->rel); -- 获取元信息        funcctx->tuple_desc = BlessTupleDesc(tupdesc); -- 函数使用的元信息        usr_ctx->map = recovery_convert_tuples_by_name(usr_ctx->reltupledesc,                funcctx->tuple_desc, "Error converting tuple descriptors!", &usr_ctx->droppedcolumn); -- 列映射        usr_ctx->scan = heap_beginscan(usr_ctx->rel, SnapshotAny, 0, NULL , NULL, 0); -- 扫描全部表数据        active_scan = heap_beginscan(usr_ctx->rel, GetActiveSnapshot(), 0, NULL , NULL, 0); -- 扫描可见数据        while ((tuplein = heap_getnext(active_scan, ForwardScanDirection)) != NULL)            hash_search(usr_ctx->active_ctid, (void*)&tuplein->t_self, HASH_ENTER, NULL); -- 缓存可见数据的 ctid
    }
    funcctx = SRF_PERCALL_SETUP(); -- 获取函数之前的上下文    usr_ctx = (pg_recovery_ctx *) funcctx->user_fctx;
get_tuple:    if ((tuplein = heap_getnext(usr_ctx->scan, ForwardScanDirection)) != NULL)    {        -- 检验表该数据是否是dead        hash_search(usr_ctx->active_ctid, (void*)&tuplein->t_self, HASH_FIND, &alive);
        tuplein = recovery_do_convert_tuple(tuplein, usr_ctx->map, alive); -- 将原表数据转换成输出格式        SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(tuplein)); -- 转换成Datum格式,返回数据    }    else    {           -- 读取完数据        heap_endscan(usr_ctx->scan); -- 结束扫描表        heap_close(usr_ctx->rel, AccessShareLock); -- 释放锁        SRF_RETURN_DONE(funcctx); --释放函数资源    }}

TupleConversionMap *recovery_convert_tuples_by_name(TupleDesc indesc,                       TupleDesc outdesc,                       const char *msg, bool *droppedcolumn){
    attrMap = recovery_convert_tuples_by_name_map(indesc, outdesc, msg, droppedcolumn); -- 处理recoveryrow/隐藏列/可见列的映射
    map->indesc = indesc;    map->outdesc = outdesc;    map->attrMap = attrMap;    map->outvalues = (Datum *) palloc(n * sizeof(Datum));    map->outisnull = (bool *) palloc(n * sizeof(bool));    map->invalues = (Datum *) palloc(n * sizeof(Datum));    map->inisnull = (bool *) palloc(n * sizeof(bool));    map->invalues[0] = (Datum) 0;    map->inisnull[0] = true;
    return map;}

HeapTuplerecovery_do_convert_tuple(HeapTuple tuple, TupleConversionMap *map, bool alive){    heap_deform_tuple(tuple, map->indesc, invalues + 1, inisnull + 1); -- 将元组拆分,提取列数据
    for (i = 0; i < outnatts; i++)    {        outvalues[i] = invalues[j]; -- 转换数据        outisnull[i] = inisnull[j]; -- 转换数据    }
    return heap_form_tuple(map->outdesc, outvalues, outisnull); -- 将列数据转换成元组}