Tuesday, February 9, 2016

How many columns in a query

Everybody knows that the limit for number of columns in an Oracle table is 1000. It is actually limit of all columns in the table, including internal ones, virtual, unused but not yet dropped and so on.

But what is the limit for a query?

Let's start with a simple table, called many_columns. It has 1000 columns, all NUMBERs, to make things easy. Columns are named COLUMN_0001 to COLUMN_1000.

And I insert 1 row into the table:

insert into many_columns(COLUMN_0001) values (1);
commit;

So what happens with an innocent query?

select m.*, n.* from many_columns m, many_columns n;

Well, nothing special - SQL*Plus is happy to return 2000 columns.

Obviously, there must an upper limit, right? At the very maximum, OCI specifies value for column count as ub2, i.e. max 65535.
However, SQL*Plus complains much sooner: the limit seems to be 8150. I added one more table - many_columns2 with just then columns. The first query to go over the limit, with 8151, fails with:

select
m01.*,
m02.*,
m03.*,
m04.*,
m05.*,
m06.*,
m07.*,
m08.*,
m10.*,
m11.*,
m12.*,
m13.*,
m14.*,
m15.*,
m16.*,
m17.*,
m18.*,
m19.*,
m20.*,
m21.*,
m22.*,
m23.*,
m24.*,
m25.*,
dummy
from
many_columns m01,
many_columns m02,
many_columns m03,
many_columns m04,
many_columns m05,
many_columns m06,
many_columns m07,
many_columns m08,
many_columns2 m10,
many_columns2 m11,
many_columns2 m12,
many_columns2 m13,
many_columns2 m14,
many_columns2 m15,
many_columns2 m16,
many_columns2 m17,
many_columns2 m18,
many_columns2 m19,
many_columns2 m20,
many_columns2 m21,
many_columns2 m22,
many_columns2 m23,
many_columns2 m24,
many_columns2 m25,
dual;

select
*
ERROR at line 1:
ORA-00913: too many values


However, in more complex situations, Oracle will complain much sooner.
select *
from   many_columns
right outer join (select count(*) c, count(*) c2 from dual) on (c=column_0001);
ERROR at line 2:
ORA-01792: maximum number of columns in a table or view is 1000

However, this is version dependent, this was in 12.1.0.2. Same test, same tables on my 11.2.0.4 environment and Oracle does not complain about this.

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