[tarantool-patches] Re: [PATCH 1/4] sql: optimize compilation of SELECT COUNT(*)
Vladislav Shpilevoy
v.shpilevoy at tarantool.org
Tue Apr 24 15:51:46 MSK 2018
See my 1 comment below. And please, rebase on the latest 2.0 - maybe some parts of your
patchset will become simpler.
On 23/04/2018 23:29, Nikita Pettik wrote:
> Originally, SQLite chooses the best index to perform COUNT operation.
> In Tarantool there is no any considerable difference between them,
> so lets don't spend time on this routine and choose always primary index,
> in case of simple query 'SELECT COUNT(*) FROM <tab>'.
> Also, patch contains codestyle fixes.
> ---
> src/box/sql/select.c | 172 ++++++++++++++++++----------------------------
> src/box/sql/vdbe.c | 2 -
> test/sql-tap/eqp.test.lua | 2 +-
> 3 files changed, 68 insertions(+), 108 deletions(-)
>
> diff --git a/src/box/sql/select.c b/src/box/sql/select.c
> index d97e466b5..0df8a71d4 100644
> --- a/src/box/sql/select.c
> +++ b/src/box/sql/select.c
> @@ -5294,25 +5296,23 @@ updateAccumulator(Parse * pParse, AggInfo * pAggInfo)
> }
> }
>
> -/*
> - * Add a single OP_Explain instruction to the VDBE to explain a simple
> - * count(*) query ("SELECT count(*) FROM pTab").
> +/**
> + * Add a single OP_Explain instruction to the VDBE to explain
> + * a simple count(*) query ("SELECT count(*) FROM <tab>").
> + *
> + * @param parse_context Current parsing context.
> + * @param table_name Name of table being queried.
> */
> #ifndef SQLITE_OMIT_EXPLAIN> static void
> -explainSimpleCount(Parse * pParse, /* Parse context */
> - Table * pTab, /* Table being queried */
> - Index * pIdx) /* Index used to optimize scan, or NULL */
> +explain_simple_count(struct Parse *parse_context, const char *table_name)
> {
> - if (pParse->explain == 2) {
> - int bCover = (pIdx != 0 && !IsPrimaryKeyIndex(pIdx));
> - char *zEqp = sqlite3MPrintf(pParse->db, "SCAN TABLE %s%s%s",
1. In Tarantool 'memtx' count() is not a scan - it is O(1) operation, that simply
gets current size of the primary index B+ tree. Maybe worth show it in the
explanation.
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