Question

我有以下设计表

car (id char(8) NOT NULL, make char(10) NOT NULL)
with NONCLUSTERED INDEX i0 ON car(make)
~80k rows of which ~2k are makes with code I need

carmake (make char(10) NOT NULL, code int NOT NULL)
with NONCLUSTERED INDEX i1 ON carmake(make)
~2k rows of which 2 have the code I want

所以我想要所有来自汽车的汽车都有carmake.code = 123

如果我这样做......我会得到下面的详细计划

select id FROM car c JOIN carmake m ON c.make = m.make where m.code = 123

请注意，它会扫描大表 - urgh！

如果我执行其中任何一项......计划显示它使用i0索引

select id from car where code ='make1' OR code ='make2'  
select id from car where code in('make1','make2')

只要您加入make表以根据代码派生品牌，查询计划就会开始创建重新格式化的工作表，然后表格扫描大表。

我无法弄清楚原因。 sybase文档指出，当要加入的表之间没有合适的密钥时，会重新格式化。在这种情况下，两个表都有make char（10）索引。我还尝试在代码上添加一个索引，这样make不会扫描表，但这仍然会导致初始重新格式化。我怀疑重新格式化导致无法使用汽车索引 - 可能是因为类型冲突和implcit转换？但为什么格式化首先发生？

计划重新格式化然后进行表扫描： -

QUERY PLAN FOR STATEMENT 1 (at line 1).
Executed in parallel by coordinating process and 3 worker processes.

STEP 1
    The type of query is INSERT.
    The update mode is direct.
    Executed by coordinating process.
    Worktable1 created for REFORMATTING.

    FROM TABLE
        make
        m
    Nested iteration.
    Table Scan.
    Forward scan.
    Positioning at start of table.
    Using I/O Size 16 Kbytes for data pages.
    With LRU Buffer Replacement Strategy for data pages.
    TO TABLE
        Worktable1.

STEP 2
    The type of query is SELECT.
    Executed in parallel by coordinating process and 3 worker processes.

    FROM TABLE
        car
        c
    Nested iteration.
    Table Scan.
    Forward scan.
    Positioning at start of table.
    Executed in parallel with a 3-way hash scan.
    Using I/O Size 16 Kbytes for data pages.
    With LRU Buffer Replacement Strategy for data pages.

    FROM TABLE
        Worktable1.
    Nested iteration.
    Using Clustered Index.
    Forward scan.
    Positioning by key.
    Using I/O Size 2 Kbytes for data pages.
    With LRU Buffer Replacement Strategy for data pages.

    Parallel network buffer merge.

The sort for Worktable1 is done in Serial

Answer 1

它真的看起来很精致。它应该采用嵌套循环连接与外表carmake和内部作为表车。您可以尝试设置'set basic_optimization on'选项。或者可能因为平行主义而采取这个计划。值得尝试关闭平行主义。

谢谢，戈帕尔

为重新格式化而创建的Sybase工作表 - 为什么？

1 个答案: