#25517: Concat() database function is not idempotent in SQLite
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Reporter: wsmith323 | Owner: nobody
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: master
Severity: Normal | Keywords: concat
| coalesce sqlite
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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When running on sqlite, with a {{{Concat()}}} expression used in the
query, the {{{ConcatPair.coalesce()}}} method is called every time the SQL
is generated. This is normal, expected, and AFAIK, correct behavior.
The problem is that {{{ConcatPair.coalesce()}}} is NOT idempotent. So,
EVERY time the SQL is generated, each expression within the
{{{ConcatPair}}} instance is wrapped with an additional `COALESCE()` SQL
function. If generated enough times, the SQL can reach a point where it
will crash the sqlite query parser.
This problem may not manifest itself in a typical request/response
context, as the SQL with the additional `COALESCE()` calls will work
identically to the original and the SQL may not be re-generated a
sufficient number of times to crash the parser.
However, in a long-running process (where this bug was found), it can be
easily triggered. For example, say I have a "base" queryset with a
{{{Concat()}}} within an {{{.annotate()}}}. I never actually evaluate this
queryset, but I use it to construct other querysets which I do evaluate.
Because all of these querysets share the same instance of
Query._annotations, evaluating ANY of these querysets will add an
additional level of COALESCE() to the SQL generated by the others.
I have a fix coded. I will submit a PR shortly.
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Ticket URL: <https://code.djangoproject.com/ticket/25517>
Django <https://code.djangoproject.com/>
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