Peng Yu created FLINK-22143:
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Summary: Flink returns less rows than expected when using limit in
SQL
Key: FLINK-22143
URL: https://issues.apache.org/jira/browse/FLINK-22143
Project: Flink
Issue Type: Bug
Components: Table SQL / Runtime
Affects Versions: 1.13.0
Reporter: Peng Yu
Fix For: 1.13.0
Flink's blink runtime returns less rows than expected when querying Hive tables
with limit.
{code:java}
// sql
select i_item_sk from tpcds_1g_snappy.item limit 5000;
{code}
Above query will return only *4998* lines in some cases.
This problem can be re-produced on below conditions:
# A Hive table with parquet format.
# Running SQL with limit using blink planner since Flink version 1.12.0
# The input table is small. (With only 1 data file in which there is only 1
row group, e.g. 1 GB of TPCDS benchmark data)
# The requested count of lines by `limit` is above the batch size (2048 by
default)
After investigation, a bug is found lying in the *LimitableBulkFormat* class.
In this class, for each batch, *numRead* will be increased *1* more than actual
count of rows returned by reader.readBatch().
The reason is that *numRead* get increased even when next() reaches then end of
current batch.
If there is only 1 input split, no more lines will be merged into the final
result.
As a result, less lines will be returned by Flink.
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