I don't mean to poke a busy thread with a possibly stupid newbie question, but
here goes.
How is this that much different than say, the SQLITE_DEFAULT_FILE_FORMAT
compile option? (Pasting it here then continuing comment below)
----Text pasted here----
SQLITE_DEFAULT_FILE_FORMAT=<1 or 4>
The default schema format number used by SQLite when creating new database
files is set by this macro. The schema formats are all very similar. The
difference between formats 1 and 4 is that format 4 understands descending
indices and has a tighter encoding for boolean values.
All versions of SQLite since 3.3.0 (2006-01-10) can read and write any
schema format between 1 and 4. But older versions of SQLite might not be able
to read formats greater than 1. So that older versions of SQLite will be able
to read and write database files created by newer versions of SQLite, the
default schema format was set to 1 for SQLite versions through 3.7.9
(2011-11-01). Beginning with version 3.7.10, the default schema format is 4.
The schema format number for a new database can be set at runtime using the
PRAGMA legacy_file_format command.
----End quoted section----
The key point when introducing something new seems to be "as long as old
versions will know they shouldn't mess with it, then it's ok." So things like
CTE's can be added to the language as the old parser will gag on them and not
try to do something wrong with them and fail. But just straight up changing the
enforcement here would be bad, because the old version wouldn't know that
something new is going on. So although the above file format option is intended
for the physical structure of the file, could for example we call file format 5
to be "same format, but will not null primary key enforced." Then old versions
would open it up, see file format 5, and throw their hands up saying "I can't
deal with this." And with new versions it wouldn't be a changeable option, it
would be "hey, once you create this database file with this option, then you're
stuck with that enforcement forever." Looking at the dates above, format 4 was
optional for 5 years before it got a promotion to default, similarly a new
value would have to be explicitly specified for n years before anyone would
have to worry about there being a "default build" that would make something
that could not be read by old versions.
I know that actually using SQLITE_DEFAULT_FILE_FORMAT for this would be very
bad and not what it's intended for. But for example, there are 20 bytes of
"Reserved for expansion. Must be zero." in the header at the moment. Do past or
current versions throw up an error if those aren't zero at the moment? Might it
be time to appropriate a byte of reserved space for new flags or an additional
number? Or would that be the start of a slippery slope?
(As a newbie I apologize if this is just plain wrong, if I just created the
sound of hundreds of foreheads smacking into their desks in unison, or if I
just re-stirred a hornets' nest)
