Thanks for the question regarding "connect by ", versi

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2 - filter("COL2"=2) Note ----- - dynamic sampling used for this statement gregs-ORA10 > create index junk_ind1 on junk ( col2 ) 2 / Index created. gregs-ORA10 > select col1 2 from junk 3 start with col2 = 2 4 connect by prior col1 = col2 5 / Execution Plan ---------------------------------------------------------- Plan hash value: 3206560888 ------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 26 | 1 (0)| 00:00:01 | |* 1 | CONNECT BY WITH FILTERING | | | | | | | 2 | TABLE ACCESS BY INDEX ROWID | JUNK | 1 | 52 | 1 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | JUNK_IND1 | 1 | | 1 (0)| 00:00:01 | | 4 | NESTED LOOPS | | | | | | | 5 | BUFFER SORT | | | | | | | 6 | CONNECT BY PUMP | | | | | | | 7 | TABLE ACCESS BY INDEX ROWID| JUNK | 1 | 26 | 1 (0)| 00:00:01 | |* 8 | INDEX RANGE SCAN | JUNK_IND1 | 1 | | 1 (0)| 00:00:01 | | 9 | TABLE ACCESS FULL | JUNK | 8 | 312 | 2 (0)| 00:00:01 | ------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - access("COL2"=PRIOR "COL1") 3 - access("COL2"=2) 8 - access("COL2"=PRIOR "COL1") Note ----- - dynamic sampling used for this statement gregs-ORA10 > analyze table junk compute statistics; Table analyzed. gregs-ORA10 > select col1 2 from junk 3 start with col2 = 2 4 connect by prior col1 = col2 5 / Execution Plan ---------------------------------------------------------- Plan hash value: 3206560888 ------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 2 | 6 | 1 (0)| 00:00:01 | |* 1 | CONNECT BY WITH FILTERING | | | | | | | 2 | TABLE ACCESS BY INDEX ROWID | JUNK | 1 | 6 | 1 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | JUNK_IND1 | 1 | | 1 (0)| 00:00:01 | | 4 | NESTED LOOPS | | | | | | | 5 | BUFFER SORT | | | | | | | 6 | CONNECT BY PUMP | | | | | | | 7 | TABLE ACCESS BY INDEX ROWID| JUNK | 2 | 6 | 1 (0)| 00:00:01 | |* 8 | INDEX RANGE SCAN | JUNK_IND1 | 1 | | 1 (0)| 00:00:01 | | 9 | TABLE ACCESS FULL | JUNK | 8 | 40 | 2 (0)| 00:00:01 | ------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): ---------------------------------------------------
1 - access("COL2"=PRIOR "COL1") 3 - access("COL2"=2) 8 - access("COL2"=PRIOR "COL1") This is and Oracle 10g (10.2.0.2.0) database, and my question is, why is Oracle doing that final FULL SCAN on JUNK ?? I just don't understand the logic - I don't know where it's coming from. I think the "START WITH" results in one Index scan, and the connect by results in the 2nd ... but I can't figure out that 3rd scan. The original query ran fine in 8i (doing 2 range scans, and no third scan at all!) Sorry ... don't mean to harass .. ;) June 5, 2006 - 10am Central time zone Bookmark | Bottom | Top Reviewer: Greg from Toronto I know you're busy, and I know you don't always see every post ... however, we're really stuck on this, I've tried the Oracle Metalink Forums ... nothing yet ... and we've continued to try a few other things .. (I'm currently reading through Jonathan's book "Cost Based Oracle Fundamentals" in the hopes of sheding some light on it .. but I'm still stumped ... If you can find some time in your busy schedule to take a peek at this one, I'd be internally grateful ... if you can't ... well .. I understand!! ;) Followup June 5, 2006 - 10am Central time zone: this is a teeny tiny table, I don't see the point - the table is too small to use the index here. True .. however ... June 6, 2006 - 1pm Central time zone Bookmark | Bottom | Top Reviewer: Greg from Toronto This is a re-created test case based on a real problem I have with a table that's 17 million rows ... Same explain is showing up in that query as this one. Statistics are all up to date ... I can also re-produce the test case with a ~17 million row table .. but I was hoping an 8 row table might be easier to work with ?? Just to be clear .. it's not that I'm going to complain about the full table scan .. (yet) .. just wondering why Oracle has decided to make that third pass ... I can't seem to understand why it would want to make a third pass on that data ... all because we added an index ?? (Note the explain with no index does 2x full scans ... but after the index is added, we're making 3x passes ... 2x index scans, + 1x full ... ) I'm only trying to understand that third pass .. Thanks! Followup June 6, 2006 - 2pm Central time zone: it is a "dummy no-op" pass, it is an artifact of the explain plan output. select col1 from junk start with col2 = 2 connect by prior col1 = col2 call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ----------
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and half hours!! thx tom , i really
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