测试Postgres表分区的HASH函数

Question

我正在使用Postgres 11，并且希望在主键为UUID的表上使用哈希分区。我知道我需要预先选择多个分区，并且主键上的哈希函数的模数将用于为每个分区分配行。

类似这样的东西：

CREATE TABLE new_table ( id uuid ) PARTITION BY HASH (id);
CREATE TABLE new_table_0 PARTITION OF new_table FOR VALUES WITH (MODULUS 3, REMAINDER 0);
CREATE TABLE new_table_1 PARTITION OF new_table FOR VALUES WITH (MODULUS 3, REMAINDER 1);
CREATE TABLE new_table_2 PARTITION OF new_table FOR VALUES WITH (MODULUS 3, REMAINDER 2);

documentation提及“分区键的哈希值”，但未指定该哈希如何发生。我想针对现有数据测试此哈希函数，以查看不同数量的分区的分布模式。像这样的东西：

SELECT unknown_partition_hash_function(id) AS hash_value, COUNT(id) AS number_of_records
FROM existing_table
GROUP BY 1

是否可以在SELECT语句中使用此哈希函数？

Answer 1

我建议在postgres / src / backend / partitioning / partbounds.c中查看PostgreSQL源代码，其中引用了可以从SQL调用的2个C函数：

/    *
     * satisfies_hash_partition
     *
     * This is an SQL-callable function for use in hash partition constraints.
     * The first three arguments are the parent table OID, modulus, and remainder.
     * The remaining arguments are the value of the partitioning columns (or
     * expressions); these are hashed and the results are combined into a single
     * hash value by calling hash_combine64.
     *
     * Returns true if remainder produced when this computed single hash value is
     * divided by the given modulus is equal to given remainder, otherwise false.
     *
     * See get_qual_for_hash() for usage.
     */
    Datum
    satisfies_hash_partition(PG_FUNCTION_ARGS)

可以在src / test / regress / sql / hash_part.sql]中找到示例。