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Improved comment on the codeDeferredSeek() routine. No code changes.

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FossilOrigin-Name: 54f49f65ac943263a1622b1efe519c8a61f30f23694fd4fec89ad2bf0e17b473
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drh
2022-02-25 13:29:56 +00:00
parent d1f0a86c12
commit 7fd6a7761f
3 changed files with 21 additions and 13 deletions
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20
src/wherecode.c
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@@ -1118,13 +1118,21 @@ static void codeCursorHint(
**
** OP_DeferredSeek $iCur $iRowid
**
** Which causes a seek on $iCur to the row with rowid $iRowid.
**
** However, if the scan currently being coded is a branch of an OR-loop and
** the statement currently being coded is a SELECT, then P3 of OP_DeferredSeek
** is set to iIdxCur and P4 is set to point to an array of integers
** containing one entry for each column of the table cursor iCur is open
** on. For each table column, if the column is the i'th column of the
** index, then the corresponding array entry is set to (i+1). If the column
** does not appear in the index at all, the array entry is set to 0.
** the statement currently being coded is a SELECT, then additional information
** is added that might allow OP_Column to omit the seek and instead do its
** lookup on the index, thus avoiding an expensive seek operation. To
** enable this optimization, the P3 of OP_DeferredSeek is set to iIdxCur
** and P4 is set to an array of integers containing one entry for each column
** in the table. For each table column, if the column is the i'th
** column of the index, then the corresponding array entry is set to (i+1).
** If the column does not appear in the index at all, the array entry is set
** to 0. The OP_Column opcode can check this array to see if the column it
** wants is in the index and if it is, it will substitute the index cursor
** and column number and continue with those new values, rather than seeking
** the table cursor.
*/
static void codeDeferredSeek(
WhereInfo *pWInfo, /* Where clause context */