Re: [**EXTERNAL**] Re: [pgsql-admin] "Soft-hitting" the 1600 column limit

To my mind, it makes perfect sense for columns to persist in the table structure when dropped…the only question I have is whether the column would survive a VACUUM FULL? i.e. if the table is rewritten after the column is dropped, would that change things?

Kav Moradhassel | Ciena
kmoradha(at)ciena(dot)com<mailto:kmoradha(at)ciena(dot)com> | 385 Terry Fox Drive | Ottawa, ON, K2K 0L1 Canada

From: Wells Oliver <wells(dot)oliver(at)gmail(dot)com>
Date: Wednesday, June 6, 2018 at 2:46 PM
To: nunks <nunks(dot)lol(at)gmail(dot)com>
Cc: Scott Ribe <scott_ribe(at)elevated-dev(dot)com>, pgsql-admin <pgsql-admin(at)postgresql(dot)org>
Subject: [**EXTERNAL**] Re: [pgsql-admin] "Soft-hitting" the 1600 column limit

Is there a pointer to some deeper explanation of this? It's news to me and kind of fascinating that dropped columns don't disappear. I did this stupid test, which obviously failed:

mydb=# create table wells.foo (col1 text, col2 text);
CREATE TABLE
mydb=# insert into wells.foo values ('a','b'),('c','d');
INSERT 0 2
mydb=# alter table wells.foo drop column col2;
ALTER TABLE
mydb=# insert into wells.foo (col1,col2) values('a','b');
ERROR: column "col2" of relation "foo" does not exist
LINE 1: insert into wells.foo (col1,col2) values('a','b');

Just curious then, in what meaningful way do dropped columns persist, what are the reasons?

On Wed, Jun 6, 2018 at 10:13 AM, nunks <nunks(dot)lol(at)gmail(dot)com<mailto:nunks(dot)lol(at)gmail(dot)com>> wrote:
Thank you all for your answers.

David and Tom's answers are more than enough to feed my argument to the developers. =)

Scott, you're right of course. My first action to reestablish their development database was to do a dump/restore, but by logging DML statements I can see the application is dropping and adding the same column over and over again, so it's a matter of time before it hangs. I hope showing this thread to the developers will make them see some sense.

@Wells, don't ask me, I have no idea, hahah, but it's a small table and the application keeps dropping/readding the column, actually. I figure the developer is using some kind of crazy ORM that drops the column when he deletes an object property and re-adds it when he sets the property back or something.

@Ervin, a vacuum full doesn't "fix" this behavior, since it's expected after all =)

collimittest=# \d add_drop_text
Table "public.add_drop_text"
Column | Type | Collation | Nullable | Default
--------+------+-----------+----------+---------
fixed | text | | |

collimittest=# vacuum FULL ANALYZE VERBOSE add_drop_text ;
INFO: vacuuming "public.add_drop_text"
INFO: "add_drop_text": found 0 removable, 0 nonremovable row versions in 0 pages
DETAIL: 0 dead row versions cannot be removed yet.
CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s.
INFO: analyzing "public.add_drop_text"
INFO: "add_drop_text": scanned 0 of 0 pages, containing 0 live rows and 0 dead rows; 0 rows in sample, 0 estimated total rows
VACUUM
collimittest=# alter table add_drop_text add column x int;
ERROR: tables can have at most 1600 columns

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“Life beats down and crushes the soul and art reminds you that you have one.”

- Stella Adler

On Wed, Jun 6, 2018 at 1:57 PM, Scott Ribe <scott_ribe(at)elevated-dev(dot)com<mailto:scott_ribe(at)elevated-dev(dot)com>> wrote:
He's *NOT* doing anything that requires 1600 columns--that was pretty much the point of his post.

--
Scott Ribe
scott_ribe(at)elevated-dev(dot)com<mailto:scott_ribe(at)elevated-dev(dot)com>
https://www.linkedin.com/in/scottribe/

> On Jun 6, 2018, at 10:53 AM, Wells Oliver <wells(dot)oliver(at)gmail(dot)com<mailto:wells(dot)oliver(at)gmail(dot)com>> wrote:
>
> let's just stop for a moment and talk about what you're doing that requires *1600 columns* because my jaw is hitting the floor.

--
Wells Oliver
wells(dot)oliver(at)gmail(dot)com<mailto:wellsoliver(at)gmail(dot)com>