Support evaluating min/max only metadata query #14845
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Assuming we have a daily ingested table that is partitioned on ds, a filter like `ds = (SELECT '2020-07-01')` is converted into an INNER JOIN, but this value is not passed to the other side of Join, which leads to full table scan. This commit will enable this value being treated as predicate, and thus we only need to read this one partition.
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highker
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presto-hive/src/test/java/com/facebook/presto/hive/TestHiveLogicalPlanner.java
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...main/src/main/java/com/facebook/presto/sql/planner/optimizations/MetadataQueryOptimizer.java
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...main/src/main/java/com/facebook/presto/sql/planner/optimizations/MetadataQueryOptimizer.java
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...main/src/main/java/com/facebook/presto/sql/planner/optimizations/MetadataQueryOptimizer.java
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| if (arguments.isEmpty()) { | ||
| return constant(null, returnType); | ||
| } |
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When will the result be null?
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The result would be null if all values are null.
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That being said, I should probably move this to be the first step of this function.
Assuming we have a daily ingested table that is partitioned on ds, one common use case is to fetch data from latest ds partition. One way to compose such a query is using a filter like `ds = (SELECT max(ds) FROM table)`. However, this filter is converted into an INNER JOIN, and will lead to a full table scan on the other side of join. Instead, this commit enables a query like `SELECT max(ds) FROM table` being evaluated at optimization time when OPTIMIZE_METADATA_QUERIES is set to true, and convert it into a ValuesNode, which could then be pushed to the other side of Join to avoid expensive full table scan.
highker
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Jul 27, 2020
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Note that enabling existing config
optimizer.optimize-metadata-queriesand session propertyoptimize_metadata_queriesmight change query result if there are metadata that refers to empty data, e.g. empty hive partition. For example, if we have two Hive ds partitions, one is2020-07-01and the other is2020-08-01. Let's assume2020-08-01is an empty partition. Then when computing without metadata optimizer, thedsrows come from data, and since2020-08-01does not have any data, it won't be appearing in the result (e.g.DISTINCT dswould only return2020-07-01). However, if metadata optimizer is enabled, thendsrows come from metastore, andDISTINCT dswould return both rows.