[SPARK-31562][SQL] Update ExpressionDescription for substring, current_date, and current_timestamp

### What changes were proposed in this pull request?

This PR intends to add entries for substring, current_date, and current_timestamp in the SQL built-in function documents. Specifically, the entries are as follows;

 - SELECT current_date;
 - SELECT current_timestamp;
 - SELECT substring('abcd' FROM 1);
 - SELECT substring('abcd' FROM 1 FOR 2);

### Why are the changes needed?

To make the SQL (built-in functions) references complete.

### Does this PR introduce any user-facing change?

<img width="1040" alt="Screen Shot 2020-04-25 at 16 51 07" src="https://user-images.githubusercontent.com/692303/80274851-6ca5ee00-8718-11ea-9a35-9ae82008cb4b.png">

<img width="974" alt="Screen Shot 2020-04-25 at 17 24 24" src="https://user-images.githubusercontent.com/692303/80275032-a88d8300-8719-11ea-92ec-95b80169ae28.png">

<img width="862" alt="Screen Shot 2020-04-25 at 17 27 48" src="https://user-images.githubusercontent.com/692303/80275114-36696e00-871a-11ea-8e39-02e93eabb92f.png">

### How was this patch tested?

Added test examples.

Closes apache#28342 from maropu/SPARK-31562.

Authored-by: Takeshi Yamamuro <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: maropu

Diff for: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -404,7 +404,7 @@ object FunctionRegistry {
     expression[Month]("month"),
     expression[MonthsBetween]("months_between"),
     expression[NextDay]("next_day"),
-    expression[CurrentTimestamp]("now", true),
+    expression[Now]("now"),
     expression[Quarter]("quarter"),
     expression[Second]("second"),
     expression[ParseToTimestamp]("to_timestamp"),

Diff for: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/UnsupportedOperationChecker.scala

@@ -19,13 +19,11 @@ package org.apache.spark.sql.catalyst.analysis
 
 import org.apache.spark.internal.Logging
 import org.apache.spark.sql.AnalysisException
-import org.apache.spark.sql.catalyst.expressions.{Attribute, AttributeReference, AttributeSet, CurrentDate, CurrentTimestamp, MonotonicallyIncreasingID}
+import org.apache.spark.sql.catalyst.expressions.{Attribute, CurrentDate, CurrentTimestamp, MonotonicallyIncreasingID, Now}
 import org.apache.spark.sql.catalyst.expressions.aggregate.AggregateExpression
-import org.apache.spark.sql.catalyst.planning.ExtractEquiJoinKeys
 import org.apache.spark.sql.catalyst.plans._
 import org.apache.spark.sql.catalyst.plans.logical._
 import org.apache.spark.sql.catalyst.streaming.InternalOutputModes
-import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.streaming.OutputMode
 
 /**
@@ -412,7 +410,7 @@ object UnsupportedOperationChecker extends Logging {
 
       subPlan.expressions.foreach { e =>
         if (e.collectLeaves().exists {
-          case (_: CurrentTimestamp | _: CurrentDate) => true
+          case (_: CurrentTimestamp | _: Now | _: CurrentDate) => true
           case _ => false
         }) {
           throwError(s"Continuous processing does not support current time operations.")

Diff for: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/datetimeExpressions.scala

@@ -28,7 +28,6 @@ import org.apache.commons.text.StringEscapeUtils
 import org.apache.spark.SparkUpgradeException
 import org.apache.spark.sql.AnalysisException
 import org.apache.spark.sql.catalyst.InternalRow
-import org.apache.spark.sql.catalyst.analysis.FunctionRegistry
 import org.apache.spark.sql.catalyst.expressions.codegen._
 import org.apache.spark.sql.catalyst.expressions.codegen.Block._
 import org.apache.spark.sql.catalyst.util.{DateTimeUtils, LegacyDateFormats, TimestampFormatter}
@@ -62,7 +61,21 @@ trait TimeZoneAwareExpression extends Expression {
  * There is no code generation since this expression should get constant folded by the optimizer.
  */
 @ExpressionDescription(
-  usage = "_FUNC_() - Returns the current date at the start of query evaluation.",
+  usage = """
+    _FUNC_() - Returns the current date at the start of query evaluation.
+
+    _FUNC_ - Returns the current date at the start of query evaluation.
+  """,
+  examples = """
+    Examples:
+      > SELECT _FUNC_();
+       2020-04-25
+      > SELECT _FUNC_;
+       2020-04-25
+  """,
+  note = """
+    The syntax without braces has been supported since 2.0.1.
+  """,
   group = "datetime_funcs",
   since = "1.5.0")
 case class CurrentDate(timeZoneId: Option[String] = None)
@@ -83,26 +96,52 @@ case class CurrentDate(timeZoneId: Option[String] = None)
   override def prettyName: String = "current_date"
 }
 
+abstract class CurrentTimestampLike() extends LeafExpression with CodegenFallback {
+  override def foldable: Boolean = true
+  override def nullable: Boolean = false
+  override def dataType: DataType = TimestampType
+  override def eval(input: InternalRow): Any = currentTimestamp()
+}
+
 /**
  * Returns the current timestamp at the start of query evaluation.
  * All calls of current_timestamp within the same query return the same value.
  *
  * There is no code generation since this expression should get constant folded by the optimizer.
  */
 @ExpressionDescription(
-  usage = "_FUNC_() - Returns the current timestamp at the start of query evaluation.",
+  usage = """
+    _FUNC_() - Returns the current timestamp at the start of query evaluation.
+
+    _FUNC_ - Returns the current timestamp at the start of query evaluation.
+  """,
+  examples = """
+    Examples:
+      > SELECT _FUNC_();
+       2020-04-25 15:49:11.914
+      > SELECT _FUNC_;
+       2020-04-25 15:49:11.914
+  """,
+  note = """
+    The syntax without braces has been supported since 2.0.1.
+  """,
   group = "datetime_funcs",
   since = "1.5.0")
-case class CurrentTimestamp() extends LeafExpression with CodegenFallback {
-  override def foldable: Boolean = true
-  override def nullable: Boolean = false
-
-  override def dataType: DataType = TimestampType
-
-  override def eval(input: InternalRow): Any = currentTimestamp()
+case class CurrentTimestamp() extends CurrentTimestampLike {
+  override def prettyName: String = "current_timestamp"
+}
 
-  override def prettyName: String =
-    getTagValue(FunctionRegistry.FUNC_ALIAS).getOrElse("current_timestamp")
+@ExpressionDescription(
+  usage = "_FUNC_() - Returns the current timestamp at the start of query evaluation.",
+  examples = """
+    Examples:
+      > SELECT _FUNC_();
+       2020-04-25 15:49:11.914
+  """,
+  group = "datetime_funcs",
+  since = "1.6.0")
+case class Now() extends CurrentTimestampLike {
+  override def prettyName: String = "now"
 }
 
 /**

Diff for: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala

@@ -1619,7 +1619,11 @@ case class StringSpace(child: Expression)
  */
 // scalastyle:off line.size.limit
 @ExpressionDescription(
-  usage = "_FUNC_(str, pos[, len]) - Returns the substring of `str` that starts at `pos` and is of length `len`, or the slice of byte array that starts at `pos` and is of length `len`.",
+  usage = """
+    _FUNC_(str, pos[, len]) - Returns the substring of `str` that starts at `pos` and is of length `len`, or the slice of byte array that starts at `pos` and is of length `len`.
+
+    _FUNC_(str FROM pos[ FOR len]]) - Returns the substring of `str` that starts at `pos` and is of length `len`, or the slice of byte array that starts at `pos` and is of length `len`.
+  """,
   examples = """
     Examples:
       > SELECT _FUNC_('Spark SQL', 5);
@@ -1628,6 +1632,12 @@ case class StringSpace(child: Expression)
        SQL
       > SELECT _FUNC_('Spark SQL', 5, 1);
        k
+      > SELECT _FUNC_('Spark SQL' FROM 5);
+       k SQL
+      > SELECT _FUNC_('Spark SQL' FROM -3);
+       SQL
+      > SELECT _FUNC_('Spark SQL' FROM 5 FOR 1);
+       k
   """,
   since = "1.5.0")
 // scalastyle:on line.size.limit

Diff for: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/finishAnalysis.scala

@@ -85,7 +85,7 @@ object ComputeCurrentTime extends Rule[LogicalPlan] {
             LocalDate.now(DateTimeUtils.getZoneId(timeZoneId)),
             DateType)
         })
-      case CurrentTimestamp() => currentTime
+      case CurrentTimestamp() | Now() => currentTime
     }
   }
 }

Diff for: sql/core/src/test/scala/org/apache/spark/sql/expressions/ExpressionInfoSuite.scala

@@ -120,6 +120,9 @@ class ExpressionInfoSuite extends SparkFunSuite with SharedSparkSession {
     val ignoreSet = Set(
       // One of examples shows getting the current timestamp
       "org.apache.spark.sql.catalyst.expressions.UnixTimestamp",
+      "org.apache.spark.sql.catalyst.expressions.CurrentDate",
+      "org.apache.spark.sql.catalyst.expressions.CurrentTimestamp",
+      "org.apache.spark.sql.catalyst.expressions.Now",
       // Random output without a seed
       "org.apache.spark.sql.catalyst.expressions.Rand",
       "org.apache.spark.sql.catalyst.expressions.Randn",

Diff for: sql/gen-sql-functions-docs.py

@@ -104,9 +104,11 @@ def _make_pretty_usage(infos):
     result.append("  <tbody>")
 
     for info in infos:
-        # Extracts (signature, description) pairs from `info.usage`, e.g.,
-        # the signature is `func(expr)` and the description is `...` in an usage `func(expr) - ...`.
-        usages = iter(re.split(r"(%s\(.*\)) - " % info.name, info.usage.strip())[1:])
+        # Extracts (signature, description) pairs from `info.usage`.
+        # Expected formats are as follows;
+        #  - `_FUNC_(...) - description`, or
+        #  - `_FUNC_ - description`
+        usages = iter(re.split(r"(%s.*) - " % info.name, info.usage.strip())[1:])
         for (sig, description) in zip(usages, usages):
             result.append("    <tr>")
             result.append("      <td>%s</td>" % sig)