feat(small): Support <slt:ignore> marker in sqllogictest for non-deterministic expected parts (apache#18857)

## Which issue does this PR close?

<!--
We generally require a GitHub issue to be filed for all bug fixes and
enhancements and this helps us generate change logs for our releases.
You can link an issue to this PR using the GitHub syntax. For example
`Closes #123` indicates that this PR will close issue #123.
-->

Part of apache#17612

## Rationale for this change

<!--
Why are you proposing this change? If this is already explained clearly
in the issue then this section is not needed.
Explaining clearly why changes are proposed helps reviewers understand
your changes and offer better suggestions for fixes.
-->
`sqllogictest`s are in general easier to maintain than rust tests,
however it's not able to test `EXPLAIN ANALYZE` results, because their
results include changing part:

(in datafusion-cli) The `elapsed_compute` measurement changes from run
to run.
```
> EXPLAIN ANALYZE SELECT * FROM generate_series(100);
+-------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| plan_type         | plan                                                                                                                                                                                             |
+-------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Plan with Metrics | LazyMemoryExec: partitions=1, batch_generators=[generate_series: start=0, end=100, batch_size=8192], metrics=[output_rows=101, elapsed_compute=74.042µs, output_bytes=64.0 KB, output_batches=1] |
|                   |                                                                                                                                                                                                  |
+-------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row(s) fetched.
Elapsed 0.006 seconds.
```

We can add a special marker to `sqllogictest` to skip those
non-deterministic parts.

## What changes are included in this PR?

<!--
There is no need to duplicate the description in the issue here but it
is sometimes worth providing a summary of the individual changes in this
PR.
-->

- Changed `sqllogictest` validator to recognize `<slt:ignore>` marker
- doc
- slt test

## Are these changes tested?

<!--
We typically require tests for all PRs in order to:
1. Prevent the code from being accidentally broken by subsequent changes
2. Serve as another way to document the expected behavior of the code

If tests are not included in your PR, please explain why (for example,
are they covered by existing tests)?
-->

## Are there any user-facing changes?

<!--
If there are user-facing changes then we may require documentation to be
updated before approving the PR.
-->

<!--
If there are any breaking changes to public APIs, please add the `api
change` label.
-->

---------

Co-authored-by: Martin Grigorov <martin-g@users.noreply.github.com>

Author: 2010YOUY01

datafusion/sqllogictest/README.md

@@ -142,6 +142,17 @@ select substr('Andrew Lamb', 1, 6), '|'
 Andrew |
 ```
 
+## Cookbook: Ignoring volatile output
+
+Sometimes parts of a result change every run (timestamps, counters, etc.). To keep the rest of the snapshot checked in, replace those fragments with the `<slt:ignore>` marker inside the expected block. During validation the marker acts like a wildcard, so only the surrounding text must match.
+
+```text
+query TT
+EXPLAIN ANALYZE SELECT * FROM generate_series(100);
+----
+Plan with Metrics LazyMemoryExec: partitions=1, batch_generators=[generate_series: start=0, end=100, batch_size=8192], metrics=[output_rows=101, elapsed_compute=<slt:ignore>, output_bytes=<slt:ignore>]
+```
+
 # Reference
 
 ## Running tests: Validation Mode

datafusion/sqllogictest/src/util.rs

@@ -82,13 +82,43 @@ pub fn df_value_validator(
     actual: &[Vec<String>],
     expected: &[String],
 ) -> bool {
+    // Support ignore marker <slt:ignore> to skip volatile parts of output.
+    const IGNORE_MARKER: &str = "<slt:ignore>";
+    let contains_ignore_marker = expected.iter().any(|line| line.contains(IGNORE_MARKER));
+
     let normalized_expected = expected.iter().map(normalizer).collect::<Vec<_>>();
     let normalized_actual = actual
         .iter()
         .map(|strs| strs.iter().join(" "))
         .map(|str| str.trim_end().to_string())
         .collect_vec();
 
+    // If ignore marker present, perform fragment-based matching on the full snapshot.
+    if contains_ignore_marker {
+        let expected_snapshot = normalized_expected.join("\n");
+        let actual_snapshot = normalized_actual.join("\n");
+        let fragments: Vec<&str> = expected_snapshot.split(IGNORE_MARKER).collect();
+        let mut pos = 0;
+        for (i, frag) in fragments.iter().enumerate() {
+            if frag.is_empty() {
+                continue;
+            }
+            if let Some(idx) = actual_snapshot[pos..].find(frag) {
+                // Edge case: The following example is expected to fail
+                // Actual - 'foo bar baz'
+                // Expected - 'bar <slt:ignore>'
+                if (i == 0) && (idx != 0) {
+                    return false;
+                }
+
+                pos += idx + frag.len();
+            } else {
+                return false;
+            }
+        }
+        return true;
+    }
+
     if log_enabled!(Warn) && normalized_actual != normalized_expected {
         warn!("df validation failed. actual vs expected:");
         for i in 0..normalized_actual.len() {
@@ -110,3 +140,20 @@ pub fn df_value_validator(
 pub fn is_spark_path(relative_path: &Path) -> bool {
     relative_path.starts_with("spark/")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // Validation should fail for the below case:
+    // Actual - 'foo bar baz'
+    // Expected - 'bar <slt:ignore>'
+    #[test]
+    fn ignore_marker_does_not_skip_leading_text() {
+        // Actual snapshot contains unexpected prefix before the expected fragment.
+        let actual = vec![vec!["foo bar baz".to_string()]];
+        let expected = vec!["bar <slt:ignore>".to_string()];
+
+        assert!(!df_value_validator(value_normalizer, &actual, &expected));
+    }
+}

datafusion/sqllogictest/test_files/explain_analyze.slt

@@ -0,0 +1,27 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+statement ok
+set datafusion.explain.analyze_level = summary;
+
+query TT
+EXPLAIN ANALYZE SELECT * FROM generate_series(100);
+----
+Plan with Metrics LazyMemoryExec: partitions=1, batch_generators=[generate_series: start=0, end=100, batch_size=8192], metrics=[output_rows=101, elapsed_compute=<slt:ignore>, output_bytes=<slt:ignore>]
+
+statement ok
+reset datafusion.explain.analyze_level;

datafusion/sqllogictest/test_files/slt_features.slt

@@ -0,0 +1,74 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+# =================================
+# Test sqllogictest runner features
+# =================================
+
+# --------------------------
+# Test `<slt:ignore>` marker
+# --------------------------
+query T
+select 'DataFusion'
+----
+<slt:ignore>
+
+query T
+select 'DataFusion'
+----
+Data<slt:ignore>
+
+query T
+select 'DataFusion'
+----
+<slt:ignore>Fusion
+
+query T
+select 'Apache DataFusion';
+----
+<slt:ignore>Data<slt:ignore>
+
+query T
+select 'DataFusion'
+----
+DataFusion<slt:ignore>
+
+query T
+select 'DataFusion'
+----
+<slt:ignore>DataFusion
+
+query T
+select 'DataFusion'
+----
+<slt:ignore>DataFusion<slt:ignore>
+
+query I
+select * from generate_series(3);
+----
+0
+1
+<slt:ignore>
+3
+
+query I
+select * from generate_series(3);
+----
+<slt:ignore>
+1
+<slt:ignore>
+<slt:ignore>