Db2: create unique index: SQL0270N Function not supported (Reason code = "1")

问题描述：db2, create unique index报错： SQL0270N  Function not supported (Reason code = "1")，示意如下：
inst105@db2a:~$ db2 "create table t1(col1 int, col2 int, col3 varchar(20))"
DB20000I  The SQL command completed successfully.
inst105@db2a:~$ db2 "create unique index idx1 on t1(col2)"
DB21034E  The command was processed as an SQL statement because it was not a 
valid Command Line Processor command.  During SQL processing it returned:
SQL0270N  Function not supported (Reason code = "1").  SQLSTATE=42997

分析：
1. 先看报错的含义：The primary key, each unique constraint, and each unique index must contain all distribution columns of the table，即唯一性索引必须包含表的所有的分布列(distribution columns)

SQL0270N  Function not supported (Reason code = "").
Explanation: 
The statement cannot be processed because it violates a restriction as
indicated by the following reason code:

1
         The primary key, each unique constraint, and each unique index
         must contain all distribution columns of the table.
 
User response: 
The action corresponding to the reason code is as follows:
1
         Correct the CREATE TABLE, ALTER TABLE or CREATE UNIQUE INDEX
         statement.  

2. distribution columns只有DPF分区表才有,先看一下表的PARTITION_MODE是H，表示是partition by Hashing

inst105@db2a:~$ db2 "select substr(tabname,1,30) as tabname, PARTITION_MODE from syscat.tables where tabname='T1'"

TABNAME                            PARTITION_MODE
------------------------------ --------------
T1                                          H             

  1 record(s) selected.

再查一下对应的分区列是COL1：
inst105@db2a:~$ db2 "select substr(COLNAME,1,30) as colname, PARTKEYSEQ from syscat.columns where tabname='T1' "

COLNAME                        PARTKEYSEQ
------------------------------ ----------
COL1                                    1
COL2                                    0
COL3                                    0

  3 record(s) selected.

db2look查看表的定义，也能进一步确认：

inst105@db2a:~$ db2look -d test -a -e -t T1
..

CONNECT TO TEST;

------------------------------------------------
-- DDL Statements for Table "INST105 "."T1"
------------------------------------------------
 
CREATE TABLE "INST105 "."T1"  (
                  "COL1" INTEGER , 
                  "COL2" INTEGER , 
                  "COL3" VARCHAR(20 OCTETS) )   
                 DISTRIBUTE BY HASH("COL1")   
                   IN "USERSPACE1"  
                 ORGANIZE BY ROW; 
 
3. 信息中心中create index的语法中也有如下介绍：
If the UNIQUE option is specified, and the table has a distribution key, the columns in the index key must be a superset of the distribution key. That is, the columns specified for a unique index key must include all the columns of the distribution key (SQLSTATE 42997).

4. 综上，原因是DPF环境，T1的distribution column是col1, 那么所有的unique index必须包含col1, 作为一种折衷方案，可以考虑如下语句
inst105@db2a:~$ db2 "create unique index idx1 on t1(col2,col1)"
DB20000I  The SQL command completed successfully.