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Improved comments on the new OP_SeekScan opcode.

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FossilOrigin-Name: 6110fdd5bb7b4ae5c065404c802ae726362ef084966b73cffe58c4bfb98689d2
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drh
2020-09-29 17:29:11 +00:00
parent c40076a8cf
commit dfbaae7572
3 changed files with 42 additions and 34 deletions
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62
src/vdbe.c
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@@ -4384,45 +4384,53 @@ seek_not_found:
}
/* Opcode: SeekScan
/* Opcode: SeekScan P1 * * * *
** Synopsis: Scan-ahead up to P1 rows
**
** This opcode is a prefix. It must be followed immediately by
** OP_SeekGE and then OP_IdxGT. This opcode should occur in no other
** context. That constraint is verified using assert() statements in
** the code.
** This opcode is a prefix opcode to OP_SeekGE. In other words, this
** opcode must be immediately followed by OP_SeekGE. Furthermore, the
** OP_SeekGE must be followed by OP_IdxGT. These constraints are
** checked by assert() statements.
**
** This opcode uses the P1 through P4 operands of the subsequent
** OP_SeekGE. In the text that follows, the operands of the subsequent
** OP_SeekGE opcode are denoted as SeekOP.P1 through SeekOP.P4. Only
** the P1 operand of this opcode is used, and it is denoted as This.P1.
**
** This opcode helps to optimize IN operators on a multi-column index
** where the IN operator is on the later terms of the index.
** where the IN operator is on the later terms of the index by avoiding
** unnecessary seeks on the btree, substituting steps to the next row
** of the b-tree instead. A correct answer is obtained if this opcode
** is omitted or is a no-op.
**
** The P3 and P4 operations of the OP_SeekGE opcode that follows this
** opcode identify an unpacked key which is the desired entry that
** we want to advance the cursor to. Call this the "target".
** The SeekGE.P3 and SeekGE.P4 operands identify an unpacked key which
** is the desired entry that we want the cursor SeekGE.P1 to be pointing
** to. Call this SeekGE.P4/P5 row the "target".
**
** If the OP_SeekGE opcode that immediately follows this opcode has
** never run before, then this opcode is a no-op and control passes
** never run before, which is to say if the SeekGE.P1 cursor is not pointing
** to a valid raow, then this opcode is a no-op and control passes
** through into the OP_SeekGE.
**
** If the subsequent OP_SeekGE opcode has run before, then that prior
** might OP_SeekGE might have left the cursor pointing any entry that
** is close to the target. This routine checks, and if possible
** bypasses the OP_SeekGE.
** If the SeekGE.P1 cursor is pointing to a valid row, then that row
** might be the target row, or it might be near and slightly before the
** target row. This opcode attempts to position the cursor on the target
** row by, perhaps stepping by invoking sqlite3BtreeStep() on the cursor
** between 0 and This.P1 times.
**
** If the cursor is past the target, jump immediately to the
** P2 of the subsequent OP_SeekGE.
** There are three possible outcomes from this opcode:<ol>
**
** If the cursor is less than the target, then step forward up to P1
** times trying to find a match. If during these steps, the
** cursor moves past the target, then jump immediately to
** the P2 of the subsequent OP_SeekGE. If a match is found, jump
** to the first instruction past the OP_IdxGT that follows the
** OP_SeekGE. (In other words, skip over the next two opcodes).
** If P1 steps are performed and the cursor is still less than the
** target, then fall through into OP_SeekGE opcode.
** <li> If after This.P1 steps, the cursor is still point to a place that
** is earlier in the btree than the target row,
** then fall through into the subsquence OP_SeekGE opcode.
**
** This opcode is an optimization. This opcode can be a no-op and
** the correct answer should still be obtained. The purpose of this
** opcode is to bypass unnecessary OP_SeekGE operations.
** <li> If the cursor is successfully moved to the target row by 0 or more
** sqlite3BtreeNext() calls, then jump to the first instruction after the
** OP_IdxGT opcode - or in other words, skip the next two opcodes.
**
** <li> If the cursor ends up past the target row (indicating the the target
** row does not exist in the btree) then jump to SeekOP.P2.
** </ol>
*/
case OP_SeekScan: {
VdbeCursor *pC;