test: Add reservoir sample e2e tests

[allenshen13]

Description

Add e2e aggregation tests verifying reservoir_sample output size, processed_count, null handling, grouping behavior, and deterministic behavior on constant inputs.

Motivation and Context

Impact

Test Plan

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Summary by Sourcery

Tests:

• Add e2e aggregation tests verifying reservoir_sample output size, processed_count, null handling, grouping behavior, and deterministic behavior on constant inputs.

[sourcery-ai]

Reviewer's Guide

Adds end-to-end tests for the reservoir_sample aggregation to validate basic deterministic properties of its output over the orders table, including processed_count behavior, sample size limits, value domain, handling of constant and null inputs, and group-by semantics.

File-Level Changes

Change	Details	Files
Add end-to-end tests for the reservoir_sample aggregation function in the aggregation test suite.	Introduce testReservoirSample JUnit test method in AbstractTestAggregations Assert processed_count equals number of processed rows with and without null inputs Verify sample cardinality when input size is greater than or less than desired sample size Check that all sampled values lie within the expected positive domain Confirm deterministic behavior when sampling from a constant input Validate processed_count semantics under GROUP BY by comparing with count(*) per group	presto-tests/src/main/java/com/facebook/presto/tests/AbstractTestAggregations.java

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[sourcery-ai]

Hey - I've found 2 issues, and left some high level feedback:

• The tests hardcode assumptions about the scale and row count of the orders table (e.g., SELECT 15000), which could make them brittle if the test dataset changes; consider expressing these expectations in terms of count(*) queries instead of fixed constants.
• Several assertions scan the full orders table independently, which may slow down the e2e suite; consider combining related checks into fewer queries or using WITH subqueries / CTEs to reuse a single aggregation where possible.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The tests hardcode assumptions about the scale and row count of the `orders` table (e.g., `SELECT 15000`), which could make them brittle if the test dataset changes; consider expressing these expectations in terms of `count(*)` queries instead of fixed constants.
- Several assertions scan the full `orders` table independently, which may slow down the e2e suite; consider combining related checks into fewer queries or using `WITH` subqueries / CTEs to reuse a single aggregation where possible.

## Individual Comments

### Comment 1
<location path="presto-tests/src/main/java/com/facebook/presto/tests/AbstractTestAggregations.java" line_range="1388" />
<code_context>
+     * specific elements. Comprehensive correctness lives in TestReservoirSampleAggregation.
+     */
+    @Test
+    public void testReservoirSample()
+    {
+        // processed_count equals the number of input rows when no initial sample is supplied.
</code_context>
<issue_to_address>
**suggestion (testing):** Add coverage for non-null initial sample and non-zero initial processed_count arguments

Current tests only call the 4‑arg `reservoir_sample` with a `NULL` initial sample and `0` `processed_count`. Please add at least one end‑to‑end test that seeds the aggregator with a non‑empty initial sample and non‑zero `processed_count`, then verifies that the final `processed_count` and sample size account for both the seed and the new rows (i.e., merge/resume behavior).

Suggested implementation:

```java
        // The sample is capped at the desired size when the input is larger.
        assertQuery(
                "SELECT cardinality(rs.sample) FROM (SELECT reservoir_sample(NULL, 0, orderkey, 10) AS rs FROM orders) t",
                "SELECT 10");

        // A non-empty initial sample and non-zero initial processed_count are incorporated into the final state.
        // Seed the aggregation with an initial sample of two elements and a processed_count of 2, then process three
        // more rows. The final processed_count should be 5, and the sample should contain five elements (the seed
        // plus the new rows, with no eviction because total < desired_sample_size).
        assertQuery(
                "SELECT rs.processed_count, cardinality(rs.sample) " +
                        "FROM (" +
                        "SELECT reservoir_sample(ARRAY[1, 2], 2, x, 5) AS rs " +
                        "FROM (VALUES 10, 20, 30) t(x)" +
                        ") u",
                "VALUES (5, 5)");

        // When the input is smaller than the desired size the entire input is kept.

```

If there are existing style or helper methods in `AbstractTestAggregations` for building VALUES queries or for asserting single-row results, you may want to adapt the new assertion to use those helpers for consistency. Also, if `reservoir_sample` has separate overloads for different types, adjust the `ARRAY[1, 2]` and `VALUES 10, 20, 30` literals to match the preferred test type (e.g., BIGINT versus INTEGER) used elsewhere in this test class.
</issue_to_address>

### Comment 2
<location path="presto-tests/src/main/java/com/facebook/presto/tests/AbstractTestAggregations.java" line_range="1405-1413" />
<code_context>
+                "SELECT cardinality(filter(rs.sample, x -> x > 0)) = cardinality(rs.sample) FROM (SELECT reservoir_sample(NULL, 0, orderkey, 20) AS rs FROM orders) t",
+                "SELECT true");
+
+        // A constant input yields a deterministic sample of that constant.
+        assertQuery(
+                "SELECT rs.sample = ARRAY[1.0E0, 1.0E0, 1.0E0] FROM (SELECT reservoir_sample(NULL, 0, 1.0E0, 3) AS rs FROM orders) t",
+                "SELECT true");
+
</code_context>
<issue_to_address>
**suggestion (testing):** Clarify the determinism assumption and/or relax the assertion on the exact array shape

This assertion hard-codes the exact sampled array `[1.0E0, 1.0E0, 1.0E0]` for a constant input, which makes the test fragile if the implementation later changes (e.g., returns fewer elements under an optimization or encodes the constant differently) while still respecting the intended contract. Consider instead asserting properties like `cardinality(rs.sample) = 3` and that all elements equal `1.0E0`. If you truly require full-array determinism for constant inputs, please add a short comment stating that this is an explicit, stable guarantee of the function.

```suggestion
        // Sampled elements are drawn from the input: every sampled orderkey is positive.
        assertQuery(
                "SELECT cardinality(filter(rs.sample, x -> x > 0)) = cardinality(rs.sample) FROM (SELECT reservoir_sample(NULL, 0, orderkey, 20) AS rs FROM orders) t",
                "SELECT true");

        // A constant input yields a sample of the constant value; verify size and contents, not exact array shape.
        assertQuery(
                "SELECT cardinality(rs.sample) = 3 AND cardinality(filter(rs.sample, x -> x = 1.0E0)) = 3 FROM (SELECT reservoir_sample(NULL, 0, 1.0E0, 3) AS rs FROM orders) t",
                "SELECT true");
```
</issue_to_address>

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[czentgr]

@allenshen13 Looks like sourcery had some good suggestion on expanding the testing? Can we take a look at that?
Also please mention that while these are added to presto-tests these tests also get executed for native from presto-native-tests through AbstractTestAggregationsNative which extends the modified class.

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