如何在 C 中的 Postgres 函数的表中 for 循环

Question

我正在考虑开发一个年龄函数，返回图的邻接矩阵。邻接矩阵存储两个节点之间的边数。检查邻接矩阵中是否存在边的复杂度为 O(1)，而空间复杂度为 O(v^2)，其中 v = 顶点数。 AGE 将每个图的顶点存储在

graph_name._ag_label_vertex

中，而边存储在

graph_name._ag_label_edge

中。

顶点表包含两列：id和properties。 id 列是整数类型，而properties 列是类似 JSON 的格式。边表包含边的 id、它来自的顶点的 id、它去的顶点的 id 以及边的属性。

示例：

SELECT * FROM "graph_name"."Vertex";
       id         | properties                                             
------------------+-------------
 844424930131971  | {}
 1407374883553281 | {}


SELECT * FROM "graph_name"."Edges";
        id        |    start_id     |      end_id      | properties 
------------------+-----------------+------------------+------------
 1125899906842625 | 844424930131971 | 1407374883553281 | {}
(1 row)

我正在考虑首先遍历顶点列，以便最初可以存储顶点ID：

[0, 1, 2, 3]
[1, 0, 0, 0]
[2, 0, 0, 0]
[3, 0, 0, 0]

之后，循环遍历edges表，如果两个节点之间存在边，则存储1（如果一对节点之间有多条边，则为每条边在已经设置的值上加1）。但我不知道如何在 C 中为 postgres 函数执行此操作。 我应该调用哪个函数来遍历表并从 id 列获取值？我虽然使用了 SPI 两次，第一次用于顶点，第二次用于边缘，但我不知道这是否是最好的接近。

在age--1.3.0.sql文件中，我将函数声明为

CREATE FUNCTION ag_catalog.get_adjacency_matrix(graph_name name)
RETURNS integer[][]
LANGUAGE c
AS 'MODULE_PATHNAME';

它的定义在src/backend/commands/graph_commands.c

PG_FUNCTION_INFO_V1(get_adjacency_matrix);
/*
* Function for returning the graph's adjacency matrix. This adjacency matrix
* stores the number of edges between two nodes. The complexity to check if an
* edge exists within an adjacency matrix is O(1), while the space complexity
* is O(v^2) with v = number of vertices.
*/
Datum get_adjacency_matrix(PG_FUNCTION_ARGS)
{
    PG_RETURN_VOID();
}

Answer 1

这不是答案，但它可以被视为来自另一个函数的指南：

通过查看

get_all_edge_labels_per_graph

在某些时候，我们可以使用这些函数来获取其中的元组，如下所示：

• ScanKeyInit：用于过滤
• table_open：用于打开表xd
• heap_getnext：用于检索小管

功能：

/*
 * Retrieves a list of all the names of a graph.
 *
 * XXX: We may want to use the cache system for this function,
 * however the cache system currently requires us to know the
 * name of the label we want.
 */
List *get_all_edge_labels_per_graph(EState *estate, Oid graph_oid)
{
    List *labels = NIL;
    ScanKeyData scan_keys[2];
    Relation ag_label;
    TableScanDesc scan_desc;
    HeapTuple tuple;
    TupleTableSlot *slot;
    ResultRelInfo *resultRelInfo;

    // setup scan keys to get all edges for the given graph oid
    ScanKeyInit(&scan_keys[1], Anum_ag_label_graph, BTEqualStrategyNumber,
                F_OIDEQ, ObjectIdGetDatum(graph_oid));
    ScanKeyInit(&scan_keys[0], Anum_ag_label_kind, BTEqualStrategyNumber,
                F_CHAREQ, CharGetDatum(LABEL_TYPE_EDGE));

    // setup the table to be scanned
    ag_label = table_open(ag_label_relation_id(), RowExclusiveLock);
    scan_desc = table_beginscan(ag_label, estate->es_snapshot, 2, scan_keys);

    resultRelInfo = create_entity_result_rel_info(estate, "ag_catalog",
                                                  "ag_label");

    slot = ExecInitExtraTupleSlot(
        estate, RelationGetDescr(resultRelInfo->ri_RelationDesc),
        &TTSOpsHeapTuple);

    // scan through the results and get all the label names.
    while(true)
    {
        Name label;
        bool isNull;
        Datum datum;

        tuple = heap_getnext(scan_desc, ForwardScanDirection);

        // no more labels to process
        if (!HeapTupleIsValid(tuple))
            break;

        ExecStoreHeapTuple(tuple, slot, false);

        datum = slot_getattr(slot, Anum_ag_label_name, &isNull);
        label = DatumGetName(datum);

        labels = lappend(labels, label);
    }

    table_endscan(scan_desc);

    destroy_entity_result_rel_info(resultRelInfo);
    table_close(resultRelInfo->ri_RelationDesc, RowExclusiveLock);

    return labels;
}

参考资料：

• https://github.com/apache/age/tree/master/src/backend/catalog/ag_label.c#L274