On 10/17/2017 01:22 PM, Hick Gunter wrote:
In our virtual table implementations, we are using the rowid to return the
location of the record in the backing store (e.g. record offset in the file
used as a backing store, offset within a shared memory section or maybe even
the memory address of the record image) and also implement fast lookup by rowid.
If you don't require such ability, you may as well return a constant, a global
counter value or a counter that is reset in the xFilter function.
So, YES you always have to implement the xRowid method.
It will only get called if your SELECT statement explicitly mentions it. No "INTEGER
PRIMARY KEY" magic is performed for virtual tables.
I think the exception is queries with OR terms. With FTS[345], if you do
something like:
CREATE VIRTUAL TABLE t1 USING fts5(x);
EXPLAIN SELECT x FROM t1 WHERE t1 MATCH 'abc' OR t1 MATCH 'def';
You can see the Rowid opcodes.
SQLite runs two separate queries on the virtual table - one with "MATCH
'abc'" and the other with "MATCH 'def'". It uses the rowids for each
matched row to avoid returning duplicates. If the xRowid method always
returned 0, then only the first set of matches would be returned
(because SQLite would deem the second set to be duplicates of the
first). Or if xRowid returned arbitrary values your results might
include duplicates. etc.
Same applies to other virtual table types.
Dan.
-----Ursprüngliche Nachricht-----
Von: sqlite-users [mailto:sqlite-users-boun...@mailinglists.sqlite.org] Im
Auftrag von dave
Gesendet: Montag, 16. Oktober 2017 21:23
An: 'SQLite mailing list' <sqlite-users@mailinglists.sqlite.org>
Betreff: [EXTERNAL] [sqlite] xRowid and read only virtual tables....
Hi, I am building a system which involves a number of virtual table
implementations. They are all read-only, but will be involved in a bunch of
joins amongst themselves. My question is this:
the documentation
http://sqlite.org/vtab.html#tabfunc2 at 2.12 xRowid seems (to my reading) to
be always required to be implemented. But does it really? Is it ever used for
read-only tables? I have never seen it invoked, and I have been blithely
ignoring implementing it, but I wonder if there is a case where it would be
invoked for a read-only query and so I am tempting fate.
I ask in particular because implementing it will be quite awkward for the
underlying implementation in my case, and I'd very much prefer to skip it.
Even a 'without rowid' table would imply specifying some primary key, which in
a few cases would also be awkward.
Thanks in advance,
-dave
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Gunter Hick | Software Engineer | Scientific Games International GmbH |
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