The signature of sqlite3_prepare_v2 is as follows:
int sqlite3_prepare_v2(
  sqlite3 *db,            /* Database handle */
  const char *zSql,       /* SQL statement, UTF-8 encoded */
  int nByte,              /* Maximum length of zSql in bytes. */
  sqlite3_stmt **ppStmt,  /* OUT: Statement handle */
  const char **pzTail     /* OUT: Pointer to unused portion of zSql */
);

Quoting from the SQLite docs[1]:
"If the caller knows that the supplied string is nul-terminated, then there is a small performance advantage to passing an nByte parameter that is the number of bytes in the input string including the nul-terminator."

sqlite3_prepare_v2 is used five places in the sqlite3 module. We can easily provide the string size in those places.

[1] https://sqlite.org/c3ref/prepare.html

Note, PR 26206 does not include statement creation in _pysqlite_connection_begin (Modules/_sqlite/connection.c). That needs further refactoring, so I'll add that in a separate PR if PR 26206 is accepted.

Regarding the maximum length of an SQL string, quoting from https://sqlite.org/limits.html:
"The current implementation will only support a string or BLOB length up to 2^31-1 or 2147483647. And some built-in functions such as hex() might fail well before that point. In security-sensitive applications it is best not to try to increase the maximum string and blob length. In fact, you might do well to lower the maximum string and blob length to something more in the range of a few million if that is possible."

The size returned from functions such as PyUnicode_AsUTF8AndSize is Py_ssize_t. I suggest checking the passed SQL string size and raising OverflowError if the SQL string is larger than SQLITE_MAX_LENGTH.

SQLITE_TOOBIG is currently mapped to sqlite3.DataError. In order to keep the current behaviour, DataError must be raised.

New changeset a384b6c by Erlend Egeberg Aasland in branch 'main':
bpo-44165: Optimise sqlite3 statement preparation by passing string size (GH-26206)
a384b6c

Erlend, check out #26206 (comment). After that we can close this issue

New changeset fbf25b8 by Erlend Egeberg Aasland in branch 'main':
bpo-44165: pysqlite_statement_create now returns a Py object, not an int (GH-26484)
fbf25b8

Erlend, check out #26206 (comment). After that we can close this issue

See msg393857: I'll add a PR for _pysqlite_connection_begin as well. After that, we can close this.

IMO, the added complexity is not worth it to change the remaining usages. Closing this issue.