When answering on review, include only chunks related to your answers.
Otherwise, letter becomes really long..

>
> > @@ -1725,9 +1727,9 @@ xferOptimization(Parse * pParse,        /* Parser context */
> >       int emptyDestTest = 0;  /* Address of test for empty pDest */
> >       int emptySrcTest = 0;   /* Address of test for empty pSrc */
> >       Vdbe *v;                /* The VDBE we are building */
> > -     int destHasUniqueIdx = 0;       /* True if pDest has a UNIQUE index */
> >       int regData, regTupleid;        /* Registers holding data and tupleid */
> >       struct session *user_session = current_session();
> > +     bool is_err_action_default = false;
>
> Again: why do you need this flag? Default action is just synonym for ABORT,
> so why should we care about it?
>
>
> It's all about conflict action priorities as I said before.
> Consider the following example:
> ```
>               CREATE TABLE t1(a INTEGER PRIMARY KEY ON CONFLICT REPLACE, b);
>               CREATE TABLE t2(a INTEGER PRIMARY KEY ON CONFLICT REPLACE, b);
>               INSERT INTO t1 VALUES (1, 1), (3, 3), (5, 5);
>               INSERT INTO t2 VALUES (2, 2), (3, 4);
>               BEGIN;
>                       INSERT INTO t2 VALUES (4, 4);
>                       INSERT INTO t2 SELECT * FROM t1;
>                       INSERT INTO t2 VALUES (10, 10);
>               COMMIT;

onError is an action of whole statement, not of index.
In your example onError == ABORT == DEFAULT.
Replace action of index is not involved in code you wrote.

>+      uint32_t src_space_id = SQLITE_PAGENO_TO_SPACEID(pSrc->tnum);
>+      struct space *src_space = space_by_id(src_space_id);
>+      uint32_t dest_space_id = SQLITE_PAGENO_TO_SPACEID(pDest->tnum);
>+      struct space *dest_space = space_by_id(dest_space_id);

Move here also assert:

assert(src_space != NULL && dest_space != NULL);

> > diff --git a/test/sql-tap/gh-3307-xfer-optimization-issue.test.lua b/test/sql-tap/gh-3307-xfer-optimization-issue.test.lua
> > new file mode 100755
> > index 0000000..e75fabc
> > --- /dev/null
> > +test:do_catchsql_test(
> > +     "xfer-optimization-1.15",
> > +     [[
> > +             DROP TABLE t1;
> > +             DROP TABLE t2;
> > +             CREATE TABLE t1(a INTEGER PRIMARY KEY, b UNIQUE);
> > +             CREATE TABLE t2(a INTEGER PRIMARY KEY, b UNIQUE);
> > +             INSERT INTO t1 VALUES (2, 2), (3, 3), (5, 5);
> > +             INSERT INTO t2 VALUES (1, 1), (4, 4);
> > +             INSERT OR ROLLBACK INTO t2 SELECT * FROM t1;
>
> INSERT OT ROLLBACK outside transaction works the same as ABORT and DEFAULT.
> So, surround it with transaction and check that it really rollbacks.
>
> There are basically almost the same tests surrounded by transactions (1.30 - 1.35).

If so, remove redundant tests pls.

> -/* Opcode: RowData P1 P2 * * *
> +/* Opcode: RowData P1 P2 * * P5
>   * Synopsis: r[P2]=data
>   *
>   * Write into register P2 the complete row content for the row at
> @@ -3984,6 +3994,8 @@ case OP_SorterData: {
>   * There is no interpretation of the data.
>   * It is just copied onto the P2 register exactly as
>   * it is found in the database file.
> + * P5 can be used in debug mode to check if xferOptimization has
> + * actually started processing.
>   *
>   * If cursor P1 is an index, then the content is the key of the row.
>   * If cursor P2 is a table, then the content extracted is the data.
> @@ -3996,6 +4008,13 @@ case OP_RowData: {
>       BtCursor *pCrsr;
>       u32 n;
> 
> +#ifdef SQLITE_TEST
> +     if (pOp->p5 == 1) {

Use named value (i.e. OPFLAG_XFER_OPT) even if they are the same.

> +
> +test:do_execsql_test(
> +     "xfer-optimization-1.37",
> +     [[
> +                     INSERT INTO t2 VALUES (10, 10);

Here (and in some other places) smth wrong with indentation, fix it pls.