Re: recommended schema diff tools?

On Fri, Apr 13, 2012 at 12:57 AM, Welty, Richard <rwelty(at)ltionline(dot)com> wrote:
> can anyone recommend an open source tool for diffing schemas?
>
> (it should go without saying that i'm looking for ddl to update production
> and QA DBs from development DBs, but i'll say it, just in case.)

We toyed with this exact issue at work. In the end, we went the other
direction, and created two files, both managed in source control: a
.sql file with everything necessary to initialize the database from
scratch, and a patch script. Every change gets done (by hand) to the
primary .sql file, and the SQL statements needed to effect the
transition (eg ALTER TABLE to add a column) get added to the patch
script. A field in our singleton configuration table records the
current patch level, so only the necessary changes will be made.

It requires some developer discipline, but it ensures that there's
always an audit trail giving the _why_ of every change, which is
something that a diff utility can never do. The patch script is quite
simple, and looks broadly like this:

patchlevel = query("select patchlevel from config")
switch (patchlevel)
{
default: print("Unknown patch level!"); break;
case 1:
print("20120216: Adding Foobar columns to Quux")
query("ALTER TABLE Quux ADD foo smallint not null default 0, ADD
bar varchar")
case 2:
... etc ...
case 42:
...
// Add new patch levels here
query("update config set patchlevel=43"); query("commit");
case 43: break;
}

Every change is thus assigned a number. The current patch level is a
safe no-op; any unrecognized number is a major error. The script is
thus safe to run on any database, and will always bring that database
up to the script's current patch level.

This has worked out far safer than attempting an after-the-event diff.
YMMV of course though.

Chris Angelico