for #2084, use 3rd parse engine to parse limit for pg

sharding-core/sharding-core-parse/sharding-core-parse-common/src/main/java/org/apache/shardingsphere/core/parse/antlr/extractor/impl/dml/select/LimitExtractor.java

@@ -47,18 +47,14 @@ public final class LimitExtractor implements OptionalSQLSegmentExtractor {
     @Override
     public Optional<LimitSegment> extract(final ParserRuleContext ancestorNode, final Map<ParserRuleContext, Integer> parameterMarkerIndexes) {
         Optional<ParserRuleContext> limitNode = ExtractorUtils.findFirstChildNode(ancestorNode, RuleName.LIMIT_CLAUSE);
-        if (!limitNode.isPresent()) {
-            return Optional.absent();
-        }
-        LimitValueSegment rowCount = extractRowCount(limitNode.get(), parameterMarkerIndexes);
-        Optional<LimitValueSegment> offset = extractOffset(limitNode.get(), parameterMarkerIndexes);
-        return offset.isPresent() ? Optional.of(new LimitSegment(rowCount, offset.get())) : Optional.of(new LimitSegment(rowCount));
+        return limitNode.isPresent()
+                ? Optional.of(new LimitSegment(extractRowCount(limitNode.get(), parameterMarkerIndexes).orNull(), extractOffset(limitNode.get(), parameterMarkerIndexes).orNull()))
+                : Optional.<LimitSegment>absent();
     }
     
-    private LimitValueSegment extractRowCount(final ParserRuleContext limitNode, final Map<ParserRuleContext, Integer> parameterMarkerIndexes) {
+    private Optional<LimitValueSegment> extractRowCount(final ParserRuleContext limitNode, final Map<ParserRuleContext, Integer> parameterMarkerIndexes) {
         Optional<ParserRuleContext> rowCountNode = ExtractorUtils.findFirstChildNode(limitNode, RuleName.LIMIT_ROW_COUNT);
-        Preconditions.checkState(rowCountNode.isPresent());
-        return extractLimitValue(rowCountNode.get(), parameterMarkerIndexes);
+        return rowCountNode.isPresent() ? Optional.of(extractLimitValue(rowCountNode.get(), parameterMarkerIndexes)) : Optional.<LimitValueSegment>absent();
     }
     
     private Optional<LimitValueSegment> extractOffset(final ParserRuleContext limitNode, final Map<ParserRuleContext, Integer> parameterMarkerIndexes) {

sharding-core/sharding-core-parse/sharding-core-parse-common/src/main/java/org/apache/shardingsphere/core/parse/antlr/filler/common/dml/LimitFiller.java

@@ -40,7 +40,9 @@ public final class LimitFiller implements SQLSegmentFiller<LimitSegment> {
     public void fill(final LimitSegment sqlSegment, final SQLStatement sqlStatement) {
         SelectStatement selectStatement = (SelectStatement) sqlStatement;
         selectStatement.setLimit(new Limit());
-        fillRowCount(sqlSegment.getRowCount(), selectStatement);
+        if (sqlSegment.getRowCount().isPresent()) {
+            fillRowCount(sqlSegment.getRowCount().get(), selectStatement);
+        }
         if (sqlSegment.getOffset().isPresent()) {
             fillOffset(sqlSegment.getOffset().get(), selectStatement);
         }

sharding-core/sharding-core-parse/sharding-core-parse-common/src/main/java/org/apache/shardingsphere/core/parse/antlr/sql/segment/dml/limit/LimitSegment.java

@@ -18,7 +18,6 @@
 package org.apache.shardingsphere.core.parse.antlr.sql.segment.dml.limit;
 
 import com.google.common.base.Optional;
-import lombok.Getter;
 import lombok.RequiredArgsConstructor;
 import org.apache.shardingsphere.core.parse.antlr.sql.segment.SQLSegment;
 
@@ -28,15 +27,19 @@ import org.apache.shardingsphere.core.parse.antlr.sql.segment.SQLSegment;
  * @author duhongjun
  */
 @RequiredArgsConstructor
-@Getter
 public final class LimitSegment implements SQLSegment {
     
     private final LimitValueSegment rowCount;
     
     private final LimitValueSegment offset;
     
-    public LimitSegment(final LimitValueSegment rowCount) {
-        this(rowCount, null);
+    /**
+     * Get row count.
+     *
+     * @return row count
+     */
+    public Optional<LimitValueSegment> getRowCount() {
+        return Optional.fromNullable(rowCount);
     }
     
     /**

sharding-core/sharding-core-parse/sharding-core-parse-postgresql/src/main/antlr4/imports/postgresql/DMLStatement.g4

@@ -149,15 +149,23 @@ havingClause
     : HAVING expr
     ;
 
-// TODO limitRowCount can be null on PG
 limitClause
-    : (LIMIT (ALL | limitRowCount)) (OFFSET limitOffset (ROW | ROWS)?)?
+    : limitRowCountSyntax_ limitOffsetSyntax_?
+    | limitOffsetSyntax_ limitRowCountSyntax_?
+    ;
+
+limitRowCountSyntax_
+    : LIMIT (ALL | limitRowCount)
     ;
 
 limitRowCount
     : numberLiterals | parameterMarker
     ;
-    
+
+limitOffsetSyntax_
+    : OFFSET limitOffset (ROW | ROWS)?
+    ;
+
 limitOffset
     : numberLiterals | parameterMarker
     ;