我一直在研究Spark如何在Parquet中存储统计信息（最小/最大），以及它如何使用信息进行查询优化。我有几个问题。第一个设置：Spark 2.1.0，下面设置了1000行的Dataframe，具有长类型和字符串类型列。不过，它们按不同的列排序。

scala> spark.sql("select id, cast(id as string) text from range(1000)").sort("id").write.parquet("/secret/spark21-sortById")
scala> spark.sql("select id, cast(id as string) text from range(1000)").sort("Text").write.parquet("/secret/spark21-sortByText")

我在parquet-tools中添加了一些代码来打印统计数据并检查生成的parque文件：

hadoop jar parquet-tools-1.9.1-SNAPSHOT.jar meta /secret/spark21-sortById/part-00000-39f7ac12-6038-46ee-b5c3-d7a5a06e4425.snappy.parquet 
file:        file:/secret/spark21-sortById/part-00000-39f7ac12-6038-46ee-b5c3-d7a5a06e4425.snappy.parquet 
creator:     parquet-mr version 1.8.1 (build 4aba4dae7bb0d4edbcf7923ae1339f28fd3f7fcf) 
extra:       org.apache.spark.sql.parquet.row.metadata = {"type":"struct","fields":[{"name":"id","type":"long","nullable":false,"metadata":{}},{"name":"text","type":"string","nullable":false,"metadata":{}}]} 

file schema: spark_schema 
--------------------------------------------------------------------------------
id:          REQUIRED INT64 R:0 D:0
text:        REQUIRED BINARY O:UTF8 R:0 D:0

row group 1: RC:5 TS:133 OFFSET:4 
--------------------------------------------------------------------------------
id:           INT64 SNAPPY DO:0 FPO:4 SZ:71/81/1.14 VC:5 ENC:PLAIN,BIT_PACKED STA:[min: 0, max: 4, num_nulls: 0]
text:         BINARY SNAPPY DO:0 FPO:75 SZ:53/52/0.98 VC:5 ENC:PLAIN,BIT_PACKED

hadoop jar parquet-tools-1.9.1-SNAPSHOT.jar meta /secret/spark21-sortByText/part-00000-3d7eac74-5ca0-44a0-b8a6-d67cc38a2bde.snappy.parquet 
file:        file:/secret/spark21-sortByText/part-00000-3d7eac74-5ca0-44a0-b8a6-d67cc38a2bde.snappy.parquet 
creator:     parquet-mr version 1.8.1 (build 4aba4dae7bb0d4edbcf7923ae1339f28fd3f7fcf) 
extra:       org.apache.spark.sql.parquet.row.metadata = {"type":"struct","fields":[{"name":"id","type":"long","nullable":false,"metadata":{}},{"name":"text","type":"string","nullable":false,"metadata":{}}]} 

file schema: spark_schema 
--------------------------------------------------------------------------------
id:          REQUIRED INT64 R:0 D:0
text:        REQUIRED BINARY O:UTF8 R:0 D:0

row group 1: RC:5 TS:140 OFFSET:4 
--------------------------------------------------------------------------------
id:           INT64 SNAPPY DO:0 FPO:4 SZ:71/81/1.14 VC:5 ENC:PLAIN,BIT_PACKED STA:[min: 0, max: 101, num_nulls: 0]
text:         BINARY SNAPPY DO:0 FPO:75 SZ:60/59/0.98 VC:5 ENC:PLAIN,BIT_PACKED

所以问题是为什么Spark，特别是2.1.0，只为数字列生成min/max，而不是字符串（BINARY）字段，即使字符串字段包含在排序中？也许我错过了一个配置？

第二个问题，是如何确认Spark正在使用最小/最大值？

scala> sc.setLogLevel("INFO")
scala> spark.sql("select * from parquet.`/secret/spark21-sortById` where id=4").show

我有很多这样的台词：

17/01/17 09:23:35 INFO FilterCompat: Filtering using predicate: and(noteq(id, null), eq(id, 4))
17/01/17 09:23:35 INFO FileScanRDD: Reading File path: file:///secret/spark21-sortById/part-00000-39f7ac12-6038-46ee-b5c3-d7a5a06e4425.snappy.parquet, range: 0-558, partition values: [empty row]
...
17/01/17 09:23:35 INFO FilterCompat: Filtering using predicate: and(noteq(id, null), eq(id, 4))
17/01/17 09:23:35 INFO FileScanRDD: Reading File path: file:///secret/spark21-sortById/part-00193-39f7ac12-6038-46ee-b5c3-d7a5a06e4425.snappy.parquet, range: 0-574, partition values: [empty row]
...

问题是看起来Spark正在扫描每个文件，即使从最小/最大值，Spark也应该能够确定只有part-00000有相关数据。或者我读错了，Spark正在跳过文件？也许Spark只能使用分区值进行数据跳过？