SELECT INTO question

[Kevin Brannen <KBrannen@xxxxxxxx>]

Hi,

 

We’re trying to understand what happened with a SELECT INTO. The problem can be see with this example:

 

# create table t1 (id int, v int);

CREATE TABLE

 

# insert into t1 (select x, x from generate_series(1, 5) as g(x));

INSERT 0 5

 

# select * from t1;

id | v

----+----

  1 |  1

  2 |  2

  3 |  3

  4 |  4

  5 |  5

(5 rows)

 

nms=# select into t2 from t1;

SELECT 5

 

# select * from t2;

--

(5 rows)

 

# select * into t3 from t1;

SELECT 5

 

# select * from t3;

id | v

----+----

  1 |  1

  2 |  2

  3 |  3

  4 |  4

  5 |  5

(5 rows)

 

As you can see on the first select into, the result in t2 is … missing, no “data” at all, unlike t3 which was the expected answer. Upon closer inspection, it was realized that the “_expression_” in the statement was left out (oops!), but instead of getting a syntax error, it worked.

 

So why did it work and why was nothing stored?

 

The only answer I’ve been able to come up with is that the _expression_ was evaluated as a “null _expression_” for each row, so it gave us 5 null rows. A small part of my brain understands that, but most of my brain goes “what?!”

 

I’ve noticed that I can also do:

 

# select from t1;

--

(5 rows)

 

That also doesn’t make sense and yet it does in a weird way. I suspect the answer revolves around some corner case in the SQL Standard.

 

So, what’s going on here?

 

Thanks,

Kevin

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