熟悉Oracle的朋友一定听过或者用过这个聚合函数vm_concat。说到这个函数，含泪的讲，给我们带来了很多故障，当然都是客户开发商的人员不了解Oracle版本变化带来的问题。
是的，实际上Oracle 11g之后就不再推荐使用这个函数了，在处理varchar2的时候会导致temp使用暴涨。
然而，很多应用程序在12c或19c中可能自定义了类似wm_concat的函数，例如 my_wm_concat。这些函数被广泛使用在应用程序中。当这些应用程序的数据库迁移到国产数据库如MogDB时，如果希望数据库层面兼容而不修改应用代码，通常迁移工具只能做语句规则替换；要么我们就需要实现函数兼容。
在开源PostgreSQL版本中有string_agg函数实现类型功能，MogDB中除了支持string_agg之外，同时还支持listagg（是的，你没有看错，兼容Oracle 12c的listagg函数)，也自带了wm_concat。但需要注意一点的是，MogDB中vm_concat函数不支持像oracle一样的synonym给个别名，需要创建函数。
这里给大家简单分享一下：
MogDB=# \da+ wm_concat
List of aggregate functions
Schema | Name | Result data type | Argument data types | Description
------------+-----------+------------------+---------------------+-------------
pg_catalog | wm_concat | text | text |
(1 row)
SELECT p.proname AS function_name,
pg_catalog.pg_get_function_result(p.oid) AS result_type,
pg_catalog.pg_get_function_arguments(p.oid) AS argument_types,
CASE
WHEN p.proisagg THEN 'agg'
ELSE 'normal'
END AS function_type
,a.aggtransfn,a.aggfinalfn
FROM pg_catalog.pg_proc p
LEFT JOIN pg_catalog.pg_namespace n ON n.oid = p.pronamespace
LEFT JOIN pg_catalog.pg_aggregate a ON a.aggfnoid = p.oid
WHERE a.aggfnoid IS NOT NULL
AND p.proname like 'wm_concat%';
function_name | result_type | argument_types | function_type | aggtransfn | aggfinalfn
---------------+-------------+----------------+---------------+----------------------+----------------------
wm_concat | text | text | agg | vm_concat_state_func | vm_concat_final_func
SELECT p.proname AS function_name,
pg_catalog.pg_get_function_result(p.oid) AS result_type,
pg_catalog.pg_get_function_arguments(p.oid) AS argument_types,
CASE
WHEN p.proisagg THEN 'agg'
ELSE 'normal'
END AS function_type
,a.aggtransfn,a.aggfinalfn
FROM pg_catalog.pg_proc p
LEFT JOIN pg_catalog.pg_namespace n ON n.oid = p.pronamespace
LEFT JOIN pg_catalog.pg_aggregate a ON a.aggfnoid = p.oid
WHERE a.aggfnoid IS NOT NULL
AND p.proname like 'string_agg%';
function_name | result_type | argument_types | function_type | aggtransfn | aggfinalfn
---------------+-------------+----------------+---------------+--------------------------+--------------------------
string_agg | text | text, text | agg | string_agg_transfn | string_agg_finalfn
string_agg | bytea | bytea, bytea | agg | bytea_string_agg_transfn | bytea_string_agg_finalfn
(2 rows)
SELECT p.proname AS function_name,
pg_catalog.pg_get_function_result(p.oid) AS result_type,
pg_catalog.pg_get_function_arguments(p.oid) AS argument_types,
CASE
WHEN p.proisagg THEN 'agg'
ELSE 'normal'
END AS function_type
,a.aggtransfn,a.aggfinalfn
FROM pg_catalog.pg_proc p
LEFT JOIN pg_catalog.pg_namespace n ON n.oid = p.pronamespace
LEFT JOIN pg_catalog.pg_aggregate a ON a.aggfnoid = p.oid
WHERE a.aggfnoid IS NOT NULL
AND p.proname like 'listagg%';
function_name | result_type | argument_types | function_type | aggtransfn | aggfinalfn
---------------+-------------+-----------------------------------+---------------+-------------------------------------+------------------
listagg | text | text, text | agg | list_agg_transfn | list_agg_finalfn
listagg | text | text | agg | list_agg_noarg2_transfn | list_agg_finalfn
listagg | text | smallint, text | agg | int2_list_agg_transfn | list_agg_finalfn
listagg | text | smallint | agg | int2_list_agg_noarg2_transfn | list_agg_finalfn
listagg | text | integer, text | agg | int4_list_agg_transfn | list_agg_finalfn
listagg | text | integer | agg | int4_list_agg_noarg2_transfn | list_agg_finalfn
listagg | text | bigint, text | agg | int8_list_agg_transfn | list_agg_finalfn
listagg | text | bigint | agg | int8_list_agg_noarg2_transfn | list_agg_finalfn
listagg | text | real, text | agg | float4_list_agg_transfn | list_agg_finalfn
listagg | text | real | agg | float4_list_agg_noarg2_transfn | list_agg_finalfn
listagg | text | double precision, text | agg | float8_list_agg_transfn | list_agg_finalfn
listagg | text | double precision | agg | float8_list_agg_noarg2_transfn | list_agg_finalfn
listagg | text | numeric, text | agg | numeric_list_agg_transfn | list_agg_finalfn
listagg | text | numeric | agg | numeric_list_agg_noarg2_transfn | list_agg_finalfn
listagg | text | date, text | agg | date_list_agg_transfn | list_agg_finalfn
listagg | text | date | agg | date_list_agg_noarg2_transfn | list_agg_finalfn
listagg | text | timestamp without time zone, text | agg | timestamp_list_agg_transfn | list_agg_finalfn
listagg | text | timestamp without time zone | agg | timestamp_list_agg_noarg2_transfn | list_agg_finalfn
listagg | text | timestamp with time zone, text | agg | timestamptz_list_agg_transfn | list_agg_finalfn
listagg | text | timestamp with time zone | agg | timestamptz_list_agg_noarg2_transfn | list_agg_finalfn
listagg | text | interval, text | agg | interval_list_agg_transfn | list_agg_finalfn
listagg | text | interval | agg | interval_list_agg_noarg2_transfn | list_agg_finalfn
(22 rows)
MogDB=# \df string_agg
List of functions
Schema | Name | Result data type | Argument data types | Type | fencedmode | propackage | prokind
------------+------------+------------------+---------------------+------+------------+------------+---------
pg_catalog | string_agg | bytea | bytea, bytea | agg | f | f | f
pg_catalog | string_agg | text | text, text | agg | f | f | f
(2 rows)
MogDB=# select wm_concat(name) from test2;
wm_concat
-----------
1,2,3
(1 row)
MogDB=# select string_agg(name,',') from test2;
string_agg
------------
1,2,3
(1 row)
MogDB=# select string_agg(name,',' order by id desc ) from test2;
select string_agg(name,',' order by id desc ) from test2;
string_agg
------------
3,2,1
(1 row)
MogDB=# select listagg(name,',') within group(order by id desc ) from test2;
select listagg(name,',') within group(order by id desc ) from test2;
listagg
---------
3,2,1
(1 row)
MogDB中的wm_concat、string_agg、listagg都可以实现类似oracle 中的wm_concat函数功能。
MogDB=# CREATE OR REPLACE FUNCTION pg_catalog.my_wm_concat_state_func (results text, val text)
MogDB-# RETURNS text
MogDB-# LANGUAGE sql COST 50 IMMUTABLE
MogDB-# AS $$ select results || ',' ||val; $$;
CREATE FUNCTION
MogDB=#
MogDB=# CREATE OR REPLACE FUNCTION pg_catalog.my_wm_concat_final_func (results text)
MogDB-# RETURNS text
MogDB-# LANGUAGE sql COST 111 IMMUTABLE
MogDB-# AS $$ select substr(results, 2); $$;
CREATE FUNCTION
MogDB=#
MogDB=# CREATE AGGREGATE pg_catalog.my_wm_concat(text)
MogDB-# (
MogDB(# sfunc = pg_catalog.my_wm_concat_state_func,
MogDB(# stype = text,
MogDB(# initcond = '',
MogDB(# FINALFUNC = pg_catalog.my_wm_concat_final_func
MogDB(# );
CREATE AGGREGATE
MogDB=#
MogDB=#
MogDB=# select my_wm_concat(name) from test2;
my_wm_concat
-----------------
aaa,bbb,ccc,ddd
(1 row)
MogDB=#
欢迎大家多多体验MogDB，并发现其中的一些乐趣！
关于聚合函数的一些更多内容，详见官方文档：https://docs.mogdb.io/zh/mogdb/v5.0/aggregate-functions
