What am I doing wrong with this MERGE / INSERT query? - sql

Question

I have a database table that I want to update with SQL. Basically, it carries a set of description information for parts of a timetable booklet, but that isn't important. Some of the data is entered already via an application, but it is time consuming and blocks of it are "boiler plate" that want to be updated each week. In some cases, I may have missed entering the data via the application, and so also want to create data automatically if it doesn't exist.
That leads me to use a MERGE query, as follows:
MERGE INTO TTP_LINE_DESCRIPTION o
USING
(SELECT DISTINCT lv.lv_nv_id TLDE_NV_ID,
lv.lv_id TLDE_LV_ID,
dir.dir_id TLDE_DIR_ID,
8 TLDE_MED_FLAG,
1 TLDE_TYPE,
0 TLDE_SORT_NO,
'Timetable valid from ' || to_char(lv.lv_valid_from,'DD/MM/YYYY') || ' until ' || nvl2(lv.lv_valid_until,to_char(lv.lv_valid_until,'DD/MM/YYYY'),'further notice') TLDE_TEXT,
0 TLDE_ALIGNMENT,
null TLDE_FONT_SIZE,
null TLDE_FONT_STYLE
FROM LINE_VERSION lv
JOIN line_point_sequence lps ON (lv.lv_id = lps.lps_lv_id)
JOIN direction dir ON (dir.dir_id = lps.lps_dir_id)
where lv.lv_nv_id=3799 and lv.lv_id=10455244) n
ON (o.TLDE_NV_ID=n.TLDE_NV_ID
and o.TLDE_LV_ID=n.TLDE_LV_ID
and o.TLDE_DIR_ID=n.TLDE_DIR_ID
and o.TLDE_TYPE=n.TLDE_TYPE
and o.TLDE_SORT_NO=n.TLDE_SORT_NO)
WHEN MATCHED THEN
UPDATE SET o.TLDE_TEXT=n.TLDE_TEXT,
o.TLDE_ALIGNMENT=n.TLDE_ALIGNMENT,
o.TLDE_FONT_SIZE=n.TLDE_FONT_SIZE,
o.TLDE_FONT_STYLE=n.TLDE_FONT_STYLE
WHEN NOT MATCHED THEN
INSERT (o.tlde_id, o.tlde_nv_id, o.tlde_lv_id, o.tlde_dir_id,
o.tlde_med_flag, o.tlde_type, o.tlde_sort_no, o.tlde_text,
o.tlde_alignment, o.tlde_font_size, o.tlde_font_style,
o.updated_by, o.updated_on, o.updated_prog)
VALUES ((select max(tld.tlde_id)+1 from TTP_LINE_DESCRIPTION tld),
n.tlde_nv_id, n.tlde_lv_id, n.tlde_dir_id, n.tlde_med_flag,
n.tlde_type, n.tlde_sort_no, n.tlde_text, n.tlde_alignment,
n.tlde_font_size, n.tlde_font_style, 'STUARTR',
SYSDATE, 'PL/SQL Developer');
There is one thing to note here, which is the WHERE clause in the SELECT DISTINCT. The lv.lv_id=10455244 is a reference that I know will force the select to return only a single pair of rows in that SELECT, so that I can limit my testing. For this purpose, 10455244 is a valid value that is NOT currently in the TTP_LINE_DESCRIPTION table.
When I use a value that is in the table, the WHEN MATCHED code executes correctly, and updates a pair of rows in 0.016s.
Running the SELECT statement on its own using the value I have shown above returns the two rows that need adding in 0.109s.
Getting the max id and adding one to it (this is the primary key) as per the first item in the VALUES line at the end takes 0s.
Finally, if I write an INSERT INTO and explicitly write all of the values that I want to write for one of the rows, I can do the INSERT of one row in 0.016s.
But put it all together and ... nothing. The execution just sits there, executing, and doesn't appear to end. Or I get nervous waiting to see if it will end. I've left it a reasonable time, and nothing seems to go in.
So what's going on, and why won't it do what I think that it should?

Answer 1

You are doing this:
create table t (id, nv_id, val) as (select 1, 101, 'A' from dual);
merge into t o
using (
select 102 nv_id, 'P' val from dual union all
select 103 nv_id, 'Q' val from dual ) n
on (o.nv_id = n.nv_id)
when matched then update set val = n.val
when not matched then
insert (o.id, o.nv_id, o.val)
values ((select max(id) + 1 from t), n.nv_id, n.val);
In my case it worked, but id for inserted rows was the same: 2. When I did rollback and added primary key constraint:
alter table t add constraint t_pk primary key(id);
merge caused error - unique constraint violated. I suspect that this is something connected to your tlde_id, maybe not constraint but something else. Generating values this way is a big no-no. If your Oracle version is 12c or higher you can change this column to auto generated
generated by default on null as identity
or in older versions use sequence and trigger (find max id, add 1 and set it as starting value for sequence)
create sequence seq_t_id start with 2;
create or replace trigger t_on_insert
before insert on t for each row
begin
select seq_t_id.nextval into :new.id from dual;
end;
Then modify your merge, either by removing id from insert clause or just insert null and trigger will set proper values:
merge into t o
using (
select 102 nv_id, 'P' val from dual union all
select 103 nv_id, 'Q' val from dual ) n
on (o.nv_id = n.nv_id)
when matched then update set val = n.val
when not matched then
insert (o.nv_id, o.val)
values (n.nv_id, n.val);
Even if this does not solve your problem you should not generate ids as max() + 1, this causes problems with concurent sessions, commits, multi inserts etc.