hash join in Sybase

Sybase 提供了对表进行hash join的命令 如下：

    select * from t1, t2
    plan "(h_join (scan t1) (scan t2) )"

ASE 15之后 sybase 默认开启 allrows_mix （之前是 allrows_oltp）
区别如下：（copy from )
Optimization Goal Details 

allrows_oltp [nested loop join] 
Nested-loop joins provide efficient access when tables are indexed on join columns. 

allrows_mix [merge joins + allrows_oltp] 
A merge join can use multiple worker processes to perform:. The scan that selects rows into a worktable, note a merge join may requires a sort.

allrows_dss [hash joins + allrows_mix] 
The hash join algorithm builds an in-memory hash table of the smaller of its targets.