tree 9e2b30ce520dae468832bc0f076f6f23bf92fff3
parent 8addf4300be33c57b79580d879c08316a47e3f90
author Qifan Chen <qchen@cloudera.com> 1592533880 -0700
committer Impala Public Jenkins <impala-public-jenkins@cloudera.com> 1597288216 +0000

IMPALA-9744: Treat corrupt table stats as missing to avoid bad plans

This work addresses the current limitation in computing the total row
count for a Hive table in a scan. The row count can be incorrectly
computed as 0, even though there exists data in the Hive table. This
is the stats corruption at table level. Similar stats corruption
exists for a partition. The row count of a table or a partition
sometime can also be -1 which indicates a missing stats situation.

In the fix, as long as no partition in a Hive table exhibits any
missing or corrupt stats, the total row count for the table is computed
from the row counts in all partitions. Otherwise, Impala looks at
the table level stats particularly the table row count.

In addition, if the table stats is missing or corrupted, Impala
estimates a row count for the table, if feasible. This row count is
the sum of the row count from the partitions with good stats, and
an estimation of the number of rows in the partitions with missing or
corrupt stats. Such estimation also applies when some partition
has corrupt stats.

One way to observe the fix is through the explain of queries scanning
Hive tables with missing or corrupted stats. The cardinality for any
full scan should be a positive value (i.e. the estimated row count),
instead of 'unavailable'.  At the beginning of the explain output,
that table is still listed in the WARNING section for potentially
corrupt table statistics.

Testing:
1. Ran unit tests with queries documented in the case against Hive
   tables with the following configrations:
   a. No stats corruption in any partitions
   b. Stats corruption in some partitions
   c. Stats corruption in all partitions
2. Added two new tests in test_compute_stats.py:
   a. test_corrupted_stats_in_partitioned_Hive_tables
   b. test_corrupted_stats_in_unpartitioned_Hive_tables
3. Fixed failures in corrupt-stats.test
4. Ran "core" test

Change-Id: I9f4c64616ff7c0b6d5a48f2b5331325feeff3576
Reviewed-on: http://gerrit.cloudera.org:8080/16098
Reviewed-by: Sahil Takiar <stakiar@cloudera.com>
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>