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Comment changes that attempt to better explain the behavior of the "seekResult"

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field on VdbeCursor objects and the seekResult parameter to
sqlite3BtreeInsert() and the OPFLAG_USESEEKRESULT flag on insert opcodes.
No changes to code.

FossilOrigin-Name: 345b46be75bdf6f9fb7367a6164d89bae11995a6
This commit is contained in:
drh
2016-11-09 20:14:34 +00:00
parent 7405fa745e
commit eaf6ae2992
5 changed files with 34 additions and 32 deletions
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21
src/vdbe.c
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@@ -4322,15 +4322,10 @@ case OP_NewRowid: { /* out2 */
** then rowid is stored for subsequent return by the
** sqlite3_last_insert_rowid() function (otherwise it is unmodified).
**
** If the OPFLAG_USESEEKRESULT flag of P5 is set and if the result of
** the last seek operation (OP_NotExists or OP_SeekRowid) was a success,
** then this
** operation will not attempt to find the appropriate row before doing
** the insert but will instead overwrite the row that the cursor is
** currently pointing to. Presumably, the prior OP_NotExists or
** OP_SeekRowid opcode
** has already positioned the cursor correctly. This is an optimization
** that boosts performance by avoiding redundant seeks.
** If the OPFLAG_USESEEKRESULT flag of P5 is set, the implementation might
** run faster by avoiding an unnecessary seek on cursor P1. However,
** the OPFLAG_USESEEKRESULT flag must only be set if there have been no prior
** seeks on the cursor or if the most recent seek used a key equal to P3.
**
** If the OPFLAG_ISUPDATE flag is set, then this opcode is part of an
** UPDATE operation. Otherwise (if the flag is clear) then this opcode
@@ -5036,9 +5031,11 @@ next_tail:
** incremented by this instruction. If the OPFLAG_NCHANGE bit is clear,
** then the change counter is unchanged.
**
** If P5 has the OPFLAG_USESEEKRESULT bit set, then the cursor must have
** just done a seek to the spot where the new entry is to be inserted.
** This flag avoids doing an extra seek.
** If the OPFLAG_USESEEKRESULT flag of P5 is set, the implementation might
** run faster by avoiding an unnecessary seek on cursor P1. However,
** the OPFLAG_USESEEKRESULT flag must only be set if there have been no prior
** seeks on the cursor or if the most recent seek used a key equivalent
** to P2.
**
** This instruction only works for indices. The equivalent instruction
** for tables is OP_Insert.