Re: Does converting an indexed varchar to text rewrite its index? Docs say so, tests say no.

[Ron <ronljohnsonjr@xxxxxxxxx>]

    Since you just built the index, and it's relatively small, maybe all
    the data is still cached.

On 1/23/20 10:55 AM, Mike Lissner wrote:

I think the docs say that if you convert a varchar to text, it'll rewrite the index, but my test doesn't seem to indicate that. Is the test or the documentation wrong?

If the docs, I'll be happy to make a fix my first contribution to postgresql. :)

Here are the docs:

(https://www.postgresql.org/docs/10/sql-altertable.html)

> [...] changing the type of an existing column will require the entire table and its indexes to be rewritten. As an exception when changing the type of an existing column, if the USING clause does not change the column contents and the old type is either binary coercible to the new type or an unconstrained domain over the new type, a table rewrite is not needed; but any indexes on the affected columns must still be rebuilt.

And the test:

postgres=# CREATE TABLE t1 (id serial PRIMARY KEY, name character varying(30));
CREATE TABLE
Time: 25.927 ms
postgres=# INSERT INTO t1 (id) SELECT generate_series(1,1000000) i;
INSERT 0 1000000
Time: 2080.416 ms (00:02.080)
postgres=# CREATE INDEX ON t1 (name);
CREATE INDEX
Time: 463.373 ms <-- Index takes ~500ms to build
postgres=# ALTER TABLE t1 ALTER COLUMN name TYPE text;
ALTER TABLE
Time: 19.698 ms <-- Alter takes 20ms to run (no rebuild, right?)

Thanks!

Mike

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