Implement Spark-compatible `xpath_string`

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Issue 类型
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技术栈
rust, xml
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databases

调研方向

First check that the shared XML evaluator from the xpath issue (#271) exists under crates/spark/src/function/xml/; this function builds on it and is blocked without it. Port Spark's XPathString / UDFXPathUtil.evalString semantics — XPath 1.0 string() conversion and number formatting — into a new function there, adding #[user_doc]. Enable and extend the stub crates/sqllogictest/test_files/xml/xpath_string.slt with the cases listed; done when that suite passes against Spark's outputs.

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描述

enhancement

Part of #271 and apache/datafusion-comet#7300.

What the function does

xpath_string(xml, path) evaluates an XPath 1.0 expression and converts the result with the XPath string()
rules (XPathConstants.STRING), giving a STRING. For a node-set this is the string value of the first node in
document order, and for an element that is the concatenation of all its descendant text. Both arguments are
strings (collation-aware from 4.0, and the result has the session default string type). path must be
foldable. The result is always nullable. Available since Spark 2.0.0.

Parsing, DTD and entity handling, namespaces, NULL handling, invalid-XML and invalid-path errors, and lazy path
compilation are shared with the family. They are specified in the xpath issue. This issue covers only the
string conversion.

Examples
SELECT xpath_string('<a><b>b</b><c>cc</c></a>','a/c');
-- cc
SELECT xpath_string('<a><b>bb</b><c>cc</c></a>', 'a');
-- bbcc
SELECT xpath_string('<a><b>bb</b><c>cc</c></a>', 'a/b');
-- bb
SELECT xpath_string('<a><b>bb</b><c>cc</c></a>', 'a/d');
-- (empty string)
SELECT xpath_string('<a><b>b1</b><b>b2</b></a>', '//b');
-- b1
SELECT xpath_string('<a><b>b1</b><b>b2</b></a>', 'a/b[1]');
-- b1
SELECT xpath_string('<a><b>b1</b><b>b2</b></a>', 'a/b[2]');
-- b2
SELECT xpath_string('<a><b>b1</b><b id="b_2">b2</b></a>', 'a/b[@id="b_2"]');
-- b2

These come from the ExpressionDescription, the Hive udf_xpath_string golden files run by
HiveCompatibilitySuite, and XPathExpressionSuite. UDFXPathUtilSuite also has these cases:

  • a/b[3]/text() gives b3;
  • a/b[4]/text() (no match) gives '';
  • a/b[2]/@k gives foo;
  • an external entity (<!ENTITY embed SYSTEM "...">) expands to ''.
Edge cases to test
  • No match gives '', not NULL (a/d above). NULL comes only from a NULL or '' argument.
  • Conversion rules from XPath 1.0 string():
    • an element gives its concatenated descendant text, including whitespace-only text nodes and CDATA;
    • comment and processing-instruction text inside the element is not included;
    • an attribute gives its value;
    • a boolean gives true or false.
  • Numbers are formatted the XPath 1.0 way, not with Java's Double.toString. Integers have no decimal point
    (sum(a/b) over 1 and 2 gives 3), and the special values are NaN, Infinity and -Infinity.
    Derive the outputs from the spec and confirm them against Spark, because the formatting of large and small
    magnitudes differs between engines.
  • XPath string functions in the path: concat, substring (1-based, with rounding rules), normalize-space,
    translate, substring-before/after, and string-length (which XPath counts in characters; check
    supplementary code points).
  • Entity and character references (&amp;, &#x1F600;) are decoded in the output. So is an internal DTD
    entity.
  • A non-literal path gives DATATYPE_MISMATCH.NON_FOLDABLE_INPUT. Malformed XML and an invalid path raise
    runtime errors (see the xpath issue).
Differences across Spark versions

None found beyond the shared ones in the xpath issue. From 4.0 the result is the session default string
type rather than plain StringType.

Implementation notes
  • Spark: XPathString in sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/xml/xpath.scala.
    It calls XPathStringEvaluator, which calls UDFXPathUtil.evalString.
  • DataFusion core has no equivalent.
  • Build it on the shared evaluator from the xpath issue in crates/spark/src/function/xml/. The existing
    crates/sqllogictest/test_files/xml/xpath_string.slt is a commented-out stub that should be enabled and
    extended. Document the function with #[user_doc].

This issue was generated by an LLM (Claude) from the Apache Spark source code and tests. Examples come from Spark's ExpressionDescription examples, test suites and .sql.out golden files, but please verify them against Spark before relying on them.

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