How to ALTER a table created with SELECT INTO LOCAL TEMPORARY TABLE...?

[VolkerBarth]

SA 11.0.1.2376

When I create a local temporary table by means of a SELECT INTO LOCAL TEMPORARY TABLE, I am not able to alter that table since ALTER TABLE returns a "table not found" error. Is this by design? If so, why?

(Business case: I often "copy" remote data into temp tables and prefer to omit the declaration of all columns and types – call it lazyness. However, I usually have to define PKs and the like afterwards.)

Example:

select * into local temporary table LT
from systable

select * from LT
-- works

alter table LT add test_col int null
-- returns "table LT not found"

[Breck_Carter]

One brute force workaround is to copy-and-edit the CREATE TABLE displayed by Foxhound or produced by dbunload. Not nearly as maintainable as SELECT * which does not have to be edited when the schema changes. Folks have asked, in the past, for the ability to "create this new table to look like that one", possibly in the old product futures discussion newsgroup.

[VolkerBarth]

@Breck: I guess the even easier "brute force" method would be to use Sybase Central to mark the table and copy its schema. That needs just a few clicks/shortcuts:)

[VolkerBarth · Accepted solution]

[Note: Edited answer to reflect Breck's and Mark's responses:]

As Breck has explained, ALTER TABLE is not only forbidden for tables created by SELECT INTO TEMPORARY TABLE but for all those kinds of local temporary tables, i.e. it applies also to

• SELECT INTO #TT
• CREATE TABLE #TT
• CREATE LOCAL TEMPORARY TABLE
• DECLARE LOCAL TEMPORARY TABLE

So there seem to be two workarounds/solutions:

1. If one relies on having an "auto-generated table" from a result set (ignoring performance drawbacks), one has to use a permanent table instead of a local temporary table, and drop that table afterwards. (That's what I have done in the past but forgot about lateron:(

   select * into PT from systable

   select * from PT -- works

   alter table PT add test_col int null -- works

2. If performance does matter and one has to use a local temporary table for that reason, one should create the table explicitly with means of CREATE/DECLARE LOCAL TEMPORARY TABLE and then use INSERT INTO ... SELECT instead.

   In that respect, SQL Central is a convenient tool to copy the table schema.

[MarkCulp · Accepted solution]

Note that there are drastic performance issues with creating/dropping permanent tables... DDL forces stored procedures to be dropped from in-memory cache and must be reloaded/parsed... cached query plans will also be dropped. This is necessary since adding/dropping tables can change how table references are bound.

[Former Member]

You can't use "ALTER TABLE" on a temporary table, but it's very well possible to do a "CREATE INDEX". I used that a few times already. 1. CREATE or DECLARE the temporary table 2. fill the table 3. create the indexes

[VolkerBarth]

Interesting point - I wasn't aware of that. As one might expect, such an index isnt' shown in the system catalog, either, and can't be dropped with DROP INDEX but is dropped automatically when the table is dropped.

[Breck_Carter]

ooooo... so why not? Why not implement ALTER TABLE and DROP INDEX and all those other things? So what if it's not in the catalog?

[Breck_Carter]

Alas, the decision to not store information about local temporary tables in the system catalog was made a long time ago in a galaxy far, far away.

One side effect is this sentence in the Help: "You cannot use ALTER TABLE on a local temporary table."

Was it a good decision? If it contributed to the great speed of temp tables in SQL Anywhere, then yes, I think so.

Does that decision cause great pain? Oh, yes... from time to time... but there has not been much hue and cry over the years. Not nearly as much anguish as with other products; e.g., endless debates and discussions of the tempdb database in [this other product] and [that one].

Would it be difficult to change? Oh, gosh, I do think it would fall into the category of "changing basic architecture"... I recently had to do one of those kinds of changes, in Foxhound, not something you consider lightly 🙂

But... you're just asking for ALTER TABLE to work on local temporary tables. In theory, you could implement that without storing information in the catalog. In theory.

[VolkerBarth]

Thanks for the pointer, I really missed that sentence in the docs:) - The decision to exclude local temporary tables from the system catalog is reasonable. The introduction of ALTER LOCAL TEMPORARY TABLE might be a nice-to-have extension, but no more, I think.

[Former Member]

If you know ahead of time that you need an extra column, why not add that column to the SELECT list of the query that created the temporary table? You can provide a value, cast it to the required data type, and provide an alias for the column name.

[VolkerBarth]

Sorry for the confusion with the added column. The original requirement was to add PRIMARY and/or UNIQUE KEYs or indexes. I have chosen the "add column" example just because I thought it was more comprehensible. For additional columns, I generally do as you suggest, so that's no problem with local temporary tables:)