Question

How can I get last autoincrement value of specific table right after I open database? It’s not last_insert_rowid() because there is no insertion transaction. In other words I want to know in advance which number autoincrement will choose when inserting new row for particular table.

Answer 1

It depends on how the autoincremented column has been defined.

If the column definition is INTEGER PRIMARY KEY AUTOINCREMENT, then SQLite will keep the largest ID in an internal table called sqlite_sequence.

If the column definition does NOT contain the keyword AUTOINCREMENT, SQLite will use its ‘regular’ routine to determine the new ID. From the documentation:

The usual algorithm is to give the newly created row a ROWID that is
one larger than the largest ROWID in the table prior to the insert. If
the table is initially empty, then a ROWID of 1 is used. If the
largest ROWID is equal to the largest possible integer
(9223372036854775807) then the database engine starts picking positive
candidate ROWIDs at random until it finds one that is not previously
used. If no unused ROWID can be found after a reasonable number of
attempts, the insert operation fails with an SQLITE_FULL error. If no
negative ROWID values are inserted explicitly, then automatically
generated ROWID values will always be greater than zero.

I remember reading that, for columns without AUTOINCREMENT, the only surefire way to determine the next ID is to VACUUM the database first; that will reset all ID counters to the largest existing ID for that table + 1. But I can’t find that quote anymore, so this may no longer be true.

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That said, I agree with slash_rick_dot that fetching auto-incremented IDs beforehand is a bad idea, especially if there’s even a remote chance that another process might write to the database at the same time.