[tarantool-patches] Re: [PATCH] sql: remove redundant goto from VDBE prologue
Vladislav Shpilevoy
v.shpilevoy at tarantool.org
Sun Jun 17 23:08:08 MSK 2018
Hello. Thanks for the patch! See 4 comments below.
On 12/06/2018 21:26, Nikita Pettik wrote:
> Structure of VDBE prologue:
>
> 0: OP_Init 0 N (address to start) 0 --|
> |-> 1: ... |
> | ... |
> | N: OP_Transaction <------------
> | N+1: (Constant expressions to be saved in registers)
> | ...
> |-- M: OP_Goto 0 1 0
>
> However, last opcode in VDBE program (i.e. OP_Goto) is generated always,
> despite the existence of OP_Transaction or constant expressions.
> Thus, VDBE program for queries like <SELECT * FROM table;> features
> redundant jump. Such useless jump wastes exectuion time (although it is
> executed once) and may affect jump prediction.
>
> This patch adds conditional branch for generating jump opcode finishing
> VDBE program: it is appended only if we need to start transaction or
> code constrant expressions.
>
> Closes #3231
> ---
> Branch: https://github.com/tarantool/tarantool/commits/np/gh-3231-remove-goto-from-vdbe-prologue
> Issue: https://github.com/tarantool/tarantool/issues/3231
>
> src/box/sql/build.c | 21 +++++++++++++++------
> 1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/src/box/sql/build.c b/src/box/sql/build.c
> index 62d687b17..66bef404c 100644
> --- a/src/box/sql/build.c
> +++ b/src/box/sql/build.c
> @@ -94,9 +94,6 @@ sqlite3FinishCoding(Parse * pParse)
1. This function looks small. Lets refactor it to Tarantool
code style. If we don't, I am afraid this commit will be lost
in 'git blame', when this function finally will be refactored
completely.
> sqlite3VdbeJumpHere(v, 0);
> if (pParse->initiateTTrans)
> sqlite3VdbeAddOp0(v, OP_TTransaction);
> - if (db->init.busy == 0)
> - sqlite3VdbeChangeP5(v, 1);
2. Why is it removed? I do not see this code moved below.
> -
> /* Code constant expressions that where factored out of inner loops */
> if (pParse->pConstExpr) {
> ExprList *pEL = pParse->pConstExpr;
> @@ -107,10 +104,22 @@ sqlite3FinishCoding(Parse * pParse)
> iConstExprReg);
> }
> }
> -
> - /* Finally, jump back to the beginning of the executable code. */
> - sqlite3VdbeGoto(v, 1);
> }
> + /*
> + * Finally, jump back to the beginning of
> + * the executable code. In fact, it is required
> + * only if some additional opcodes are generated.
> + * Otherwise, it would be useless jump:
> + *
> + * 0: OP_Init 0 vdbe_end ...
> + * 1: ...
> + * ...
> + * vdbe_end: OP_Goto 0 1 ...
> + */
> + if (pParse->pConstExpr != NULL || pParse->initiateTTrans)
> + sqlite3VdbeGoto(v, 1);
> + else
> + sqlite3VdbeChangeP2(v, 0, 1);
3. As far as I see, P2 in OP_Init is 1 already when we are here. It is
not? See allocVdbe function. P2 == 1 by default, and here it can be changed to
goto to ttrans.
> }
>
> /* Get the VDBE program ready for execution
>
4. Can we test the new VDBE plan using EXPLAIN? I am wondering why all
plan changes are not tested using EXPLAIN.
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