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[[creating-instance]] ColumnStat holds the <> of a table column (as part of the[table statistics] in a metastore).

[[statistics]] .Column Statistics [cols="1,2",options="header",width="100%"] |=== | Name | Description

| [[distinctCount]] distinctCount | Number of distinct values

| [[min]] min | Minimum value

| [[max]] max | Maximum value

| [[nullCount]] nullCount | Number of null values

| [[avgLen]] avgLen | Average length of the values

| [[maxLen]] maxLen | Maximum length of the values

| [[histogram]] histogram | Histogram of values (as Histogram which is empty by default) |===

ColumnStat is computed (and <>) using[ANALYZE TABLE COMPUTE STATISTICS FOR COLUMNS] SQL command (that SparkSqlAstBuilder[translates] to[AnalyzeColumnCommand] logical command).

[source, scala]

val cols = "id, p1, p2" val analyzeTableSQL = s"ANALYZE TABLE t1 COMPUTE STATISTICS FOR COLUMNS $cols" spark.sql(analyzeTableSQL)

ColumnStat may optionally hold the <> which is empty by default. With spark.sql.statistics.histogram.enabled configuration property turned on ANALYZE TABLE COMPUTE STATISTICS FOR COLUMNS SQL command generates column (equi-height) histograms.

NOTE: spark.sql.statistics.histogram.enabled is off by default.

You can inspect the column statistics using[DESCRIBE EXTENDED] SQL command.

scala> sql("DESC EXTENDED t1 id").show
|info_name     |info_value|
|col_name      |id        |
|data_type     |int       |
|comment       |NULL      |
|min           |0         |
|max           |1         |
|num_nulls     |0         |
|distinct_count|2         |
|avg_col_len   |4         |
|max_col_len   |4         |
|histogram     |NULL      | <-- no histogram (spark.sql.statistics.histogram.enabled off)

ColumnStat is part of the[statistics of a table].

[source, scala]

// Make sure that you ran ANALYZE TABLE (as described above) val db = spark.catalog.currentDatabase val tableName = "t1" val metadata = spark.sharedState.externalCatalog.getTable(db, tableName) val stats = metadata.stats.get

scala> :type stats org.apache.spark.sql.catalyst.catalog.CatalogStatistics

val colStats = stats.colStats scala> :type colStats Map[String,org.apache.spark.sql.catalyst.plans.logical.ColumnStat]

ColumnStat is <> (serialized) while persisting the table (statistics) to a metastore.

[source, scala]

scala> :type colStats Map[String,org.apache.spark.sql.catalyst.plans.logical.ColumnStat]

val colName = "p1"

val p1stats = colStats(colName) scala> :type p1stats org.apache.spark.sql.catalyst.plans.logical.ColumnStat

import org.apache.spark.sql.types.DoubleType val props = p1stats.toMap(colName, dataType = DoubleType) scala> println(props) Map(distinctCount -> 2, min -> 0.0, version -> 1, max -> 1.4, maxLen -> 8, avgLen -> 8, nullCount -> 0)

ColumnStat is <> (deserialized) when HiveExternalCatalog is requested for restoring table statistics from properties (from a Hive Metastore).

[source, scala]

scala> :type props Map[String,String]

scala> println(props) Map(distinctCount -> 2, min -> 0.0, version -> 1, max -> 1.4, maxLen -> 8, avgLen -> 8, nullCount -> 0)

import org.apache.spark.sql.types.StructField val p1 = $"p1".double

import org.apache.spark.sql.catalyst.plans.logical.ColumnStat val colStatsOpt = ColumnStat.fromMap(table = "t1", field = p1, map = props)

scala> :type colStatsOpt Option[org.apache.spark.sql.catalyst.plans.logical.ColumnStat]

ColumnStat is also <> when JoinEstimation is requested to estimateInnerOuterJoin for Inner, Cross, LeftOuter, RightOuter and FullOuter joins.

[source, scala]

val tableName = "t1"

// Make the example reproducible import org.apache.spark.sql.catalyst.TableIdentifier val tid = TableIdentifier(tableName) val sessionCatalog = spark.sessionState.catalog sessionCatalog.dropTable(tid, ignoreIfNotExists = true, purge = true)

// CREATE TABLE t1 Seq((0, 0, "zero"), (1, 1, "one")). toDF("id", "p1", "p2"). write. saveAsTable(tableName)

// As we drop and create immediately we may face problems with unavailable partition files // Invalidate cache spark.sql(s"REFRESH TABLE $tableName")

// Use ANALYZE TABLE...FOR COLUMNS to compute column statistics // that saves them in a metastore (aka an external catalog) val df = spark.table(tableName) val allCols = df.columns.mkString(",") val analyzeTableSQL = s"ANALYZE TABLE t1 COMPUTE STATISTICS FOR COLUMNS $allCols" spark.sql(analyzeTableSQL)

// Fetch the table metadata (with column statistics) from a metastore val metastore = spark.sharedState.externalCatalog val db = spark.catalog.currentDatabase val tableMeta = metastore.getTable(db, table = tableName)

// The column statistics are part of the table statistics val colStats = tableMeta.stats.get.colStats

scala> :type colStats Map[String,org.apache.spark.sql.catalyst.plans.logical.ColumnStat]

scala> { case (name, cs) => s"$name: $cs" }.foreach(println) // the output may vary id: ColumnStat(2,Some(0),Some(1),0,4,4,None) p1: ColumnStat(2,Some(0),Some(1),0,4,4,None) p2: ColumnStat(2,None,None,0,4,4,None)

NOTE: ColumnStat does not support <> and <> metrics for binary (i.e. Array[Byte]) and string types.

=== [[toExternalString]] Converting Value to External/Java Representation (per Catalyst Data Type) -- toExternalString Internal Method

[source, scala]

toExternalString(v: Any, colName: String, dataType: DataType): String


NOTE: toExternalString is used exclusively when ColumnStat is requested for <>.

=== [[supportsHistogram]] supportsHistogram Method

[source, scala]

supportsHistogram(dataType: DataType): Boolean


NOTE: supportsHistogram is used when...FIXME

=== [[toMap]] Converting ColumnStat to Properties (ColumnStat Serialization) -- toMap Method

[source, scala]

toMap(colName: String, dataType: DataType): Map[String, String]

toMap converts <> to the <>.

[[properties]] .ColumnStat.toMap's Properties [cols="1,2",options="header",width="100%"] |=== | Key | Value

| version | 1

| distinctCount | <>

| nullCount | <>

| avgLen | <>

| maxLen | <>

| min | <> of <>

| max | <> of <>

| histogram | Serialized version of <> (using HistogramSerializer.serialize) |===

NOTE: toMap adds min, max, histogram entries only if they are available.

NOTE: Interestingly, colName and dataType input parameters bring no value to toMap itself, but merely allow for a more user-friendly error reporting when <> min and max column statistics.

toMap is used when HiveExternalCatalog is requested for hive/converting table statistics to properties (before persisting them as part of table metadata in a Hive metastore).

=== [[fromMap]] Re-Creating Column Statistics from Properties (ColumnStat Deserialization) -- fromMap Method

[source, scala]

fromMap(table: String, field: StructField, map: Map[String, String]): Option[ColumnStat]

fromMap creates a ColumnStat by fetching <> of every <> from the input map.

fromMap returns None when recovering column statistics fails for whatever reason.

WARN Failed to parse column statistics for column [fieldName] in table [table]

NOTE: Interestingly, table input parameter brings no value to fromMap itself, but merely allows for a more user-friendly error reporting when parsing column statistics fails.

fromMap is used when HiveExternalCatalog is requested for hive/restoring table statistics from properties (from a Hive Metastore).

=== [[rowToColumnStat]] Creating Column Statistics from InternalRow (Result of Computing Column Statistics) -- rowToColumnStat Method

[source, scala]

rowToColumnStat( row: InternalRow, attr: Attribute, rowCount: Long, percentiles: Option[ArrayData]): ColumnStat

rowToColumnStat <> a ColumnStat from the input row and the following positions:

[start=0] . <> . <> . <> . <> . <> . <>

If the 6th field is not empty, rowToColumnStat uses it to create <>.

NOTE: rowToColumnStat is used exclusively when AnalyzeColumnCommand is[executed] (to[compute the statistics for specified columns]).

=== [[statExprs]] statExprs Method

[source, scala]

statExprs( col: Attribute, conf: SQLConf, colPercentiles: AttributeMap[ArrayData]): CreateNamedStruct


NOTE: statExprs is used when...FIXME

Last update: 2020-11-07