----------------------------------------------------------------------- WORKLOG TASK -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- TASK...........: Add DS-MRR support for clustered primary keys CREATION DATE..: Sat, 12 Jun 2010, 08:23 SUPERVISOR.....: Igor IMPLEMENTOR....: COPIES TO......: CATEGORY.......: Client-BackLog TASK ID........: 121 (http://askmonty.org/worklog/?tid=121) VERSION........: Benchmarks-3.0 STATUS.........: Un-Assigned PRIORITY.......: 60 WORKED HOURS...: 0 ESTIMATE.......: 0 (hours remain) ORIG. ESTIMATE.: 0 PROGRESS NOTES: -=-=(Psergey - Sat, 12 Jun 2010, 08:39)=-=- High-Level Specification modified. --- /tmp/wklog.121.old.538 2010-06-12 08:39:46.000000000 +0000 +++ /tmp/wklog.121.new.538 2010-06-12 08:39:46.000000000 +0000 @@ -1 +1,16 @@ +Basic idea: DS-MRR scan should be done as follows: +1. Sort incoming keys +2. Use the sorted keys to do a disk-ordered retrieval + +Unresolved questions: + +* The "sort incoming keys" part is trivial when we have only equality ranges. + If we allow ranges of arbitrary form ( ncluding ranges with one endpoint + being infinity or ranges overlapping with one another), sorting becomes + non-trival. Do we need to support this case or support only equality ranges? + +* Can/should we use the fact rowid=={clustered PK value} for InnoDB's clustered + PKs? + +* Do we support scanning on a prefix of clustered PK? (seems to be yes?) DESCRIPTION: Currently, DS-MRR doesn't support operation over clustered primary keys. The reason for this was that - Clustered primary keys are stored in disk order and so, if the sequence of ranges is ordered, the reads will already go in disk order (and so DS-MRR's step of re-ordering reads is not necessary). - Within DS-MRR implementation, the "get rowids from keys" step is not necessary when using clustered primary key, because in InnoDB/XtraDB clustered primary key values are the rowids. However, with BKA making the MRR calls, there are cases where DS-MRR over clustered primary key is beneficial: * BKA may provide lookup keys that have duplicates and/or are in arbitrary order. In that case, DS-MRR implementation may sort the key values and order them, so that it hits the disk in key order. * When running multi-table join with high @@join_cache_level value (and so, linked join buffers), lack of MRR implementation causes the chain of linked join buffers to break. (TODO and so what? Is that really a problem?) * TODO anything else? HIGH-LEVEL SPECIFICATION: Basic idea: DS-MRR scan should be done as follows: 1. Sort incoming keys 2. Use the sorted keys to do a disk-ordered retrieval Unresolved questions: * The "sort incoming keys" part is trivial when we have only equality ranges. If we allow ranges of arbitrary form ( ncluding ranges with one endpoint being infinity or ranges overlapping with one another), sorting becomes non-trival. Do we need to support this case or support only equality ranges? * Can/should we use the fact rowid=={clustered PK value} for InnoDB's clustered PKs? * Do we support scanning on a prefix of clustered PK? (seems to be yes?) ESTIMATED WORK TIME ESTIMATED COMPLETION DATE ----------------------------------------------------------------------- WorkLog (v3.5.9)