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Overview
| Comment: | Enhance the comment on whereLoopAddBtree(). No changes to code. |
|---|---|
| Downloads: | Tarball | ZIP archive | SQL archive |
| Timelines: | family | ancestors | descendants | both | trunk |
| Files: | files | file ages | folders |
| SHA1: |
b22dd165da227a52d88b17a91e80a170 |
| User & Date: | drh 2014-07-22 12:05:32 |
Context
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2014-07-22
| ||
| 14:42 | Expire prepared statements after running ANALYZE. check-in: b083a961 user: drh tags: trunk | |
| 12:05 | Enhance the comment on whereLoopAddBtree(). No changes to code. check-in: b22dd165 user: drh tags: trunk | |
| 00:40 | For the OR-optimization, avoid generating OP_OpenRead opcodes that reopen exactly the same index. check-in: b67a6e33 user: drh tags: trunk | |
Changes
Changes to src/where.c.
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**
** cost = nSeek * (log(nRow) + K * nVisit) // covering index
** cost = nSeek * (log(nRow) + (K+3.0) * nVisit) // non-covering index
**
** Normally, nSeek is 1. nSeek values greater than 1 come about if the
** WHERE clause includes "x IN (....)" terms used in place of "x=?". Or when
** implicit "x IN (SELECT x FROM tbl)" terms are added for skip-scans.
*/
static int whereLoopAddBtree(
WhereLoopBuilder *pBuilder, /* WHERE clause information */
Bitmask mExtra /* Extra prerequesites for using this table */
){
WhereInfo *pWInfo; /* WHERE analysis context */
Index *pProbe; /* An index we are evaluating */
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** ** cost = nSeek * (log(nRow) + K * nVisit) // covering index ** cost = nSeek * (log(nRow) + (K+3.0) * nVisit) // non-covering index ** ** Normally, nSeek is 1. nSeek values greater than 1 come about if the ** WHERE clause includes "x IN (....)" terms used in place of "x=?". Or when ** implicit "x IN (SELECT x FROM tbl)" terms are added for skip-scans. ** ** The estimated values (nRow, nVisit, nSeek) often contain a large amount ** of uncertainty. For this reason, scoring is designed to pick plans that ** "do the least harm" if the estimates are inaccurate. For example, a ** log(nRow) factor is omitted from a non-covering index scan in order to ** bias the scoring in favor of using an index, since the worst-case ** performance of using an index is far better than the worst-case performance ** of a full table scan. */ static int whereLoopAddBtree( WhereLoopBuilder *pBuilder, /* WHERE clause information */ Bitmask mExtra /* Extra prerequesites for using this table */ ){ WhereInfo *pWInfo; /* WHERE analysis context */ Index *pProbe; /* An index we are evaluating */ |