Bruce,
Considering the activity on the lists (at least recently and, I think,
historically) about postgres not casting (usually integer) constant
values across types, could there be a mention of this made in the FAQ?
It seems like a logical case for inclusion under 4.8:
http://www.postgresql.org/docs/faqs/FAQ.html#4.8
I was thinking something like the following:
Also note that 7.x versions of postgres will not automatically cast
constant data in certain queries such that an index would be used. For
example, if you have the following:
CREATE TABLE index_breaker (
bigintcol int8 primary key
some_data text
);
The following query is liikely to perform a sequential scan:
SELECT some_data FROM index_breaker WHERE bigintcol = 42;
postgres will interpret the constant value as a basic int and will thus
not use the index (implicitly created by the primary key) on the
bigintcol column.
There are some workarounds for this issue [per Tom Lane]:
1. Always quote your constants:
... WHERE bigintcol = '42';
Similarly, constants can be explicitly cast:
... WHERE bigintcol = int8( 42 )
2. Use a prepared statement:
PREPARE foo(bigint) AS ... WHERE bigintcol = $1;
EXECUTE foo(42);
3. Use parameterized statements in extended-query mode (essentially the
same idea as #2, but at the protocol level). This doesn't help for
pure SQL scripts, but is very workable when coding against libpq or
JDBC. Among other things it gets you out of worrying about SQL
injection attacks when your parameter values come from untrusted
sources.
Technical improvements to wording are welcome. But I think this is
worth adding to the docs somewhere.
Thanks!
-tfo
