refactor(query): reuse matchAllRegex (#18146)

matchAllRegex is a global variable containing the precompiled regex that
matches ".+".

Prior to this commit, it was used in only one place and we called its
.Copy() method.

According to the docs, .Copy() is no longer needed for safe concurrent
access:

  Deprecated: In earlier releases, when using a Regexp in multiple
  goroutines, giving each goroutine its own copy helped to avoid lock
  contention. As of Go 1.12, using Copy is no longer necessary to avoid
  lock contention. Copy may still be appropriate if the reason for its
  use is to make two copies with different Longest settings.

Since we require Go 1.13 or later now and we're not calling the
Longest() method, this patch removes the .Copy() call.

Now that we have a reusable matchAllRegex value, this patch then
replaces all instances of regexp.MustCompile(`.+`) with matchAllRegex.
This will elminate runtime regex compilations.

query/statement_rewriter.go

@@ -7,6 +7,8 @@ import (
 	"github.com/influxdata/influxql"
 )
 
+var matchAllRegex = regexp.MustCompile(`.+`)
+
 // RewriteStatement rewrites stmt into a new statement, if applicable.
 func RewriteStatement(stmt influxql.Statement) (influxql.Statement, error) {
 	switch stmt := stmt.(type) {
@@ -61,7 +63,7 @@ func rewriteShowFieldKeyCardinalityStatement(stmt *influxql.ShowFieldKeyCardinal
 	// Use all field keys, if zero.
 	if len(stmt.Sources) == 0 {
 		stmt.Sources = influxql.Sources{
-			&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: regexp.MustCompile(`.+`)}},
+			&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: matchAllRegex}},
 		}
 	}
 
@@ -123,7 +125,7 @@ func rewriteShowMeasurementCardinalityStatement(stmt *influxql.ShowMeasurementCa
 	// Use all measurements, if zero.
 	if len(stmt.Sources) == 0 {
 		stmt.Sources = influxql.Sources{
-			&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: regexp.MustCompile(`.+`)}},
+			&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: matchAllRegex}},
 		}
 	}
 
@@ -196,7 +198,7 @@ func rewriteShowSeriesCardinalityStatement(stmt *influxql.ShowSeriesCardinalityS
 	// Use all measurements, if zero.
 	if len(stmt.Sources) == 0 {
 		stmt.Sources = influxql.Sources{
-			&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: regexp.MustCompile(`.+`)}},
+			&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: matchAllRegex}},
 		}
 	}
 
@@ -278,7 +280,7 @@ func rewriteShowTagValuesCardinalityStatement(stmt *influxql.ShowTagValuesCardin
 	// Use all measurements, if zero.
 	if len(stmt.Sources) == 0 {
 		stmt.Sources = influxql.Sources{
-			&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: regexp.MustCompile(`.+`)}},
+			&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: matchAllRegex}},
 		}
 	}
 
@@ -366,7 +368,7 @@ func rewriteShowTagKeyCardinalityStatement(stmt *influxql.ShowTagKeyCardinalityS
 	// Use all measurements, if zero.
 	if len(stmt.Sources) == 0 {
 		stmt.Sources = influxql.Sources{
-			&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: regexp.MustCompile(`.+`)}},
+			&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: matchAllRegex}},
 		}
 	}
 
@@ -480,7 +482,7 @@ func rewriteSourcesCondition(sources influxql.Sources, cond influxql.Expr) influ
 
 func rewriteSources2(sources influxql.Sources, database string) influxql.Sources {
 	if len(sources) == 0 {
-		sources = influxql.Sources{&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: matchAllRegex.Copy()}}}
+		sources = influxql.Sources{&influxql.Measurement{Regex: &influxql.RegexLiteral{Val: matchAllRegex}}}
 	}
 	for _, source := range sources {
 		switch source := source.(type) {
@@ -492,5 +494,3 @@ func rewriteSources2(sources influxql.Sources, database string) influxql.Sources
 	}
 	return sources
 }
-
-var matchAllRegex = regexp.MustCompile(`.+`)