terrymanu commented on issue #34976:
URL:
https://github.com/apache/shardingsphere/issues/34976#issuecomment-3583950296
### Understanding
- You want to know whether master supports limit '2'::int and note that
placeholders in LIMIT need proper handling during rewrite.
### Root Cause
- In PostgreSQL/openGauss visitors,
visitSelectLimitValue/visitSelectOffsetValue/visitSelectFetchValue call
Long.parseLong(((ExpressionSegment) astNode).getText()), accepting only plain
numbers or bare parameters.
- When LIMIT uses type casts or simple expressions ('2'::int, ?::int),
getText() is not a pure number, so parsing throws an exception before rewrite.
- A placeholder wrapped in a cast is not recognized as a
ParameterMarkerExpressionSegment, so its index is lost and rewrite cannot bind
it correctly.
### Analysis (code-level fix direction)
- Update both PostgreSQL and openGauss visitors to unwrap the expression
tree, preserve parameters, and fold constants instead of blindly parseLong.
- Suggested helper (add to both visitors; can be a shared private method):
```java
private LimitValueSegment toLimitValue(final ParserRuleContext ctx,
final ExpressionSegment segment) {
if (segment instanceof ParameterMarkerExpressionSegment) {
return new
ParameterMarkerLimitValueSegment(ctx.getStart().getStartIndex(),
ctx.getStop().getStopIndex(),
((ParameterMarkerExpressionSegment)
segment).getParameterMarkerIndex());
}
if (segment instanceof TypeCastExpression) {
return toLimitValue(ctx, ((TypeCastExpression)
segment).getExpression());
}
if (segment instanceof LiteralExpressionSegment) {
Object literals = ((LiteralExpressionSegment)
segment).getLiterals();
return new
NumberLiteralLimitValueSegment(ctx.getStart().getStartIndex(),
ctx.getStop().getStopIndex(),
Long.parseLong(literals.toString()));
}
// Optionally extend to UnaryExpression / BinaryOperationExpression
constant folding
throw new SQLParsingException("Unsupported LIMIT expression: " +
segment.getText());
}
@Override
public ASTNode visitSelectLimitValue(final SelectLimitValueContext ctx) {
if (null != ctx.ALL()) {
return null;
}
ExpressionSegment expr = (ExpressionSegment) visit(ctx.cExpr());
return toLimitValue(ctx, expr);
}
```
- Key points:
- Preserve parameters: ?::int should become
ParameterMarkerLimitValueSegment; the index remains intact for rewrite.
- Type casts/constant expressions: allow '2'::int to fold into a
numeric literal, avoiding parse errors.
- Other complex expressions: throw a clear “unsupported LIMIT
expression” error rather than a NumberFormatException.
- Tests to add:
- PostgreSQL/openGauss parser UTs for limit '2'::int, limit ?::int,
limit '2'::int offset '1'::int, asserting LimitSegment rowCount/offset types
and values (numeric vs parameter index).
- Optional SQL rewrite UT ensuring limit ?::int keeps the parameter
list intact and the placeholder count/order unchanged.
### Conclusion
- Master still has this gap; it only supports plain numbers or bare
parameters in LIMIT.
- Community contributors are warmly invited to submit a PR implementing
the above parsing changes and tests—we’ll gladly review and help land it.
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