Selecting tables with one record in Oracle - sql

Question

select table_name,
to_number(extractvalue(xmltype(dbms_xmlgen.getxml('select
count(*) c from '||owner||'.'||table_name)),'/ROWSET/ROW/C')) as count
from all_tables
I would like to get those with one record in the table.

Answer 1

The query crashes only when the filter predicate is applied. It's probably a bug in some query rewrite optimization. If you wrap the query in a block with the materialize hint, it seems to bypass this behavior.
with workaround as(
select /*+ materialize */
owner
,table_name
,to_number(extractvalue(xmltype(dbms_xmlgen.getxml('select count(*) c from ' || owner || '.' || table_name || ' where rownum <= 2')),'/ROWSET/ROW/C')) as row_count
from all_tables
where owner = '<your-schema>'
)
select owner, table_name, row_count
from workaround
where row_count = 1;
I also found some potential to improve the performance of this query. If you only want tables with exactly one record, there is really no need to count every single record in the table. If you add the predicate rownum <= 2 Oracle will stop scanning as soon as it has found two records. So the count will be either:
0, meaning empty table
1, meaning exactly one record
2, meaning more than 1 record
Edit to show how the optimization work:
-- Creating tables
create table t0(c number);
create table t1(c number);
create table t2(c number);
create table t3(c number);
insert into t1 values(1);
insert into t2 values(1);
insert into t2 values(2);
insert into t3 values(1);
insert into t3 values(2);
insert into t3 values(3);
commit;
SQL:
/*
|| Without rownum you can filter on any rowcount you want
*/
select *
from (select 'T0' as t, count(*) as row_count from t0 union all
select 'T1' as t, count(*) as row_count from t1 union all
select 'T2' as t, count(*) as row_count from t2 union all
select 'T3' as t, count(*) as row_count from t3
)
where row_count = 1 -- Return tables having exactly 1 record.
;
/*
|| With rownum <= 1 Oracle will stop counting after it found one row.
|| So the rowcount will be either 0 or 1.
|| row_count = 0 means that the table is empty
|| row_count = 1 means that the table is NOT empty.
||
|| The Rownum predicate prevents us from knowing if there are 2,3,4 or 5 million records.
*/
select *
from (select 'T0' as t, count(*) as row_count from t0 where rownum <= 1 union all
select 'T1' as t, count(*) as row_count from t1 where rownum <= 1 union all
select 'T2' as t, count(*) as row_count from t2 where rownum <= 1 union all
select 'T3' as t, count(*) as row_count from t3 where rownum <= 1
)
where row_count = 1 -- Return tables having at least one record
;
/*
|| With rownum <= 2 Oracle will stop counting after it found two rows.
|| So the rowcount will be either 0, 1 or 2.
|| row_count = 0 means that the table is empty
|| row_count = 1 means that the table has exactly 1 record
|| row_count = 2 means that the table has more than 1 record
||
|| The Rownum predicate prevents us from knowing if there are exactly two records, or 3,4,5 etcetera
*/
select *
from (select 'T0' as t, count(*) as row_count from t0 where rownum <= 2 union all
select 'T1' as t, count(*) as row_count from t1 where rownum <= 2 union all
select 'T2' as t, count(*) as row_count from t2 where rownum <= 2 union all
select 'T3' as t, count(*) as row_count from t3 where rownum <= 2
)
where row_count = 1 -- Return tables having exactly one record
;

Answer 2

If your query is giving the data you want, but you just want to see the results with count equal to 1, you can wrap the query like this:
select * from (
select table_name,
to_number(extractvalue(xmltype(dbms_xmlgen.getxml('select
count(*) c from '||owner||'.'||table_name)),'/ROWSET/ROW/C')) as count
from all_tables
) where count = 1

Answer 3

SELECT * FROM
(
SELECT table_name,
TO_NUMBER(EXTRACTVALUE(XMLTYPE(dbms_xmlgen.getxml('select count(*) c from '||OWNER||'.'||table_name)),'/ROWSET/ROW/C')) AS cnt
FROM all_tables
)
WHERE cnt = 1;
or
SELECT * FROM
(
SELECT table_name,
TO_NUMBER(EXTRACTVALUE(XMLTYPE(dbms_xmlgen.getxml('select count(*) c from '||OWNER||'.'||table_name)),'/ROWSET/ROW/C')) AS cnt
FROM all_tables
) slct
GROUP BY slct.table_name
HAVING slct.cnt = 1;