On 1/23/20 8:55 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?)*
I going to say it is the exception to the exception, in that in Postgres
varchar and text are essentially the same type.
FYI there is a reindex going on:
test=> set client_min_messages = debug1;
test=> CREATE TABLE t1 (id serial PRIMARY KEY, name character varying(30));
LOG: statement: CREATE TABLE t1 (id serial PRIMARY KEY, name character
varying(30));
DEBUG: CREATE TABLE will create implicit sequence "t1_id_seq" for
serial column "t1.id"
DEBUG: CREATE TABLE / PRIMARY KEY will create implicit index "t1_pkey"
for table "t1"
DEBUG: building index "t1_pkey" on table "t1" serially
CREATE TABLE
test=> INSERT INTO t1 (id) SELECT generate_series(1,1000000) i;
LOG: statement: INSERT INTO t1 (id) SELECT generate_series(1,1000000) i;
INSERT 0 1000000
test=> CREATE INDEX ON t1 (name);
LOG: statement: CREATE INDEX ON t1 (name);
DEBUG: building index "t1_name_idx" on table "t1" with request for 1
parallel worker
CREATE INDEX
test=> ALTER TABLE t1 ALTER COLUMN name TYPE text;
LOG: statement: ALTER TABLE t1 ALTER COLUMN name TYPE text;
DEBUG: building index "pg_toast_37609_index" on table "pg_toast_37609"
serially
ALTER TABLE
Thanks!
Mike
**
--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
