MDEV-40820 Wrong results: SELECT DISTINCT / GROUP BY returns duplicate rows on a RANGE-partitioned table when served by a covering index range scan - #5712
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…e rows on a RANGE-partitioned table when served by a covering index range scan SELECT DISTINCT or GROUP BY can return duplicate values from a RANGE- partitioned table when a covering index range scan incorrectly bypasses the merge of partition scans. ha_partition::can_skip_merging_scans() checks only the current multi- range prefix. Later ranges can have different prefix values, so the partition outputs do not have the ordering required to skip the priority-queue merge. Check every multi-range entry before bypassing the partition-scan merge. Require both endpoints to bind the complete unordered prefix and to agree on its bytes. Require that prefix to be the same across all ranges; otherwise keep the normal merge. The regression uses two date prefixes across several partitions and checks that SELECT DISTINCT returns each date exactly once. Bug report: https://jira.mariadb.org/browse/MDEV-40820
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SELECT DISTINCT or GROUP BY can return duplicate values from a RANGE-partitioned table when a covering index range scan incorrectly bypasses the merge of partition scans.
Bug report: https://jira.mariadb.org/browse/MDEV-40820
Root cause
ha_partition::can_skip_merging_scans() checks only the current multi-range prefix. Later ranges can have different prefix values, so the partition outputs do not have the ordering required to skip the priority-queue merge.
Changes
Regression coverage
The regression uses two date prefixes across several partitions and checks that SELECT DISTINCT returns each date exactly once.
mysql-test/main/partition_mrr_distinct.testmysql-test/main/partition_mrr_distinct.resultValidation
On
12.3at881b64e8add374802e1871814dd0a3a673a9b3e6:main.partition_mrr_distinctfailed on the unchanged target branch and passed with this fix.main.func_group,main.group_by,main.select.