[DRAFT] fix: Use CTE in Enrollment analytics queries [DHIS-16705]

dhis-2/CTE-README.md

@@ -0,0 +1,257 @@
+# Enrollment CTE refactoring
+
+## Introduction
+
+In DHIS2, retrieving Enrollment data often requires additional attributes—counts, last recorded values, or other derived fields—from associated Event tables. 
+Historically, we used correlated subqueries in the main SELECT list and in the WHERE clause. 
+However, these correlated subqueries pose two significant problems:
+
+- **Performance Degradation in PostgreSQL**
+Repeated scanning of event tables for each enrollment can be a major bottleneck.
+- **Incompatibility with Certain Analytics Engines**
+Engines like ClickHouse or Doris do not support queries that reference outer fields in subqueries (e.g., ax.enrollment in a subquery). 
+This prevents the original correlated subquery approach from working at all in those environments.
+
+To address both of these issues, we refactored the query to use Common Table Expressions (CTEs). 
+By computing results upfront in separate CTEs and then joining the results, we eliminate the need for correlated subqueries, 
+thereby improving performance and ensuring compatibility with more analytics engines.
+
+## The Original Query (Correlated Subqueries)
+
+```sql
+SELECT
+    enrollment,
+    trackedentity,
+    enrollmentdate,
+    occurreddate,
+    storedby,
+    createdbydisplayname,
+    lastupdatedbydisplayname,
+    lastupdated,
+    ST_AsGeoJSON(enrollmentgeometry),
+    longitude,
+    latitude,
+    ouname,
+    ounamehierarchy,
+    oucode,
+    enrollmentstatus,
+    ax."ou",
+    ax."Bpx0589u8y0",
+    (
+      SELECT COUNT((occurreddate IS NOT NULL))
+      FROM analytics_event_m3xtlkyblki AS subax
+      WHERE enrollment = ax.enrollment
+        AND ps = 'CWaAcQYKVpq'
+    ) AS "d6Sr0B2NJYv",
+    (
+      SELECT "Qvb7NExMqjZ"
+      FROM analytics_event_M3xtLkYBlKI
+      WHERE eventstatus != 'SCHEDULE'
+        AND enrollment = ax.enrollment
+        AND ps = 'uvMKOn1oWvd'
+      ORDER BY occurreddate DESC, created DESC
+      LIMIT 1
+    ) AS "uvMKOn1oWvd.Qvb7NExMqjZ",
+    ax."coaSpbzZiTB"
+FROM analytics_enrollment_m3xtlkyblki AS ax
+WHERE
+    enrollmentdate >= '2021-01-01'
+    AND enrollmentdate < '2022-01-01'
+    AND ax."uidlevel1" = 'ImspTQPwCqd'
+    AND (
+      SELECT "fADIatyOu2g"
+      FROM analytics_event_M3xtLkYBlKI
+      WHERE eventstatus != 'SCHEDULE'
+        AND enrollment = ax.enrollment
+        AND ps = 'uvMKOn1oWvd'
+      ORDER BY occurreddate DESC, created DESC
+      LIMIT 1
+    ) IS NULL
+ORDER BY "lastupdated" DESC NULLS LAST
+LIMIT 101 OFFSET 0;
+```
+
+## The Refactored Query (Using CTEs)
+
+```sql
+WITH pi_d6sr0b2njyv AS (
+  SELECT
+    enrollment,
+    COUNT((occurreddate IS NOT NULL)) AS value
+  FROM analytics_event_m3xtlkyblki
+  WHERE ps = 'CWaAcQYKVpq'
+  GROUP BY enrollment
+),
+ps_uvmkon1owvd_uvmkon1owvd_qvb7nexmqjz AS (
+  SELECT
+    enrollment,
+    "Qvb7NExMqjZ" AS value,
+    ROW_NUMBER() OVER (
+      PARTITION BY enrollment
+      ORDER BY occurreddate DESC, created DESC
+    ) AS rn
+  FROM analytics_event_M3xtLkYBlKI
+  WHERE eventstatus != 'SCHEDULE'
+    AND ps = 'uvMKOn1oWvd'
+),
+uvmkon1owvd_fadiatyou2g AS (
+  SELECT
+    enrollment,
+    "fADIatyOu2g" AS value
+  FROM (
+    SELECT
+      enrollment,
+      "fADIatyOu2g",
+      ROW_NUMBER() OVER (
+        PARTITION BY enrollment
+        ORDER BY occurreddate DESC, created DESC
+      ) AS rn
+    FROM analytics_event_m3xtlkyblki
+    WHERE eventstatus != 'SCHEDULE'
+      AND ps = 'uvMKOn1oWvd'
+  ) ranked
+  WHERE rn = 1
+)
+SELECT
+  ax.enrollment,
+  ax.trackedentity,
+  ax.enrollmentdate,
+  ax.occurreddate,
+  ax.storedby,
+  ax.createdbydisplayname,
+  ax.lastupdatedbydisplayname,
+  ax.lastupdated,
+  ST_AsGeoJSON(enrollmentgeometry),
+  ax.longitude,
+  ax.latitude,
+  ax.ouname,
+  ax.ounamehierarchy,
+  ax.oucode,
+  ax.enrollmentstatus,
+  ax."ou",
+  ax."Bpx0589u8y0",
+  COALESCE(mahfm.value, 0) AS d6Sr0B2NJYv,
+  rvwqm_0.value AS "uvMKOn1oWvd.Qvb7NExMqjZ",
+  ax."coaSpbzZiTB"
+FROM analytics_enrollment_m3xtlkyblki AS ax
+LEFT JOIN pi_d6sr0b2njyv AS mahfm
+  ON mahfm.enrollment = ax.enrollment
+LEFT JOIN ps_uvmkon1owvd_uvmkon1owvd_qvb7nexmqjz AS rvwqm_0
+  ON rvwqm_0.enrollment = ax.enrollment
+  AND rvwqm_0.rn = 1
+LEFT JOIN uvmkon1owvd_fadiatyou2g AS cxylo
+  ON cxylo.enrollment = ax.enrollment
+WHERE
+    enrollmentdate >= '2021-01-01'
+    AND enrollmentdate < '2022-01-01'
+    AND ax."uidlevel1" = 'ImspTQPwCqd'
+    AND cxylo.value IS NULL
+ORDER BY "lastupdated" DESC NULLS LAST
+LIMIT 101 OFFSET 0;
+```
+
+### Explanation of the CTE Approach
+
+1. `pi_d6sr0b2njyv`
+
+- Aggregates the count of events per enrollment (`where ps = 'CWaAcQYKVpq'`).
+- Eliminates the need for a correlated subquery counting `occurreddate`.
+
+2. `ps_uvmkon1owvd_uvmkon1owvd_qvb7nexmqjz`
+
+- Retrieves `Qvb7NExMqjZ` for each enrollment via a window function (`ROW_NUMBER()`) to find the most recent record.
+- Eliminates the need for a subquery that grabbed the top 1 event by `occurreddate DESC, created DESC`.
+
+3. `uvmkon1owvd_fadiatyou2g`
+
+- Retrieves the latest `"fADIatyOu2g"` value (again using `ROW_NUMBER()`).
+- Replaces the subquery used in the original `WHERE` clause to check if `"fADIatyOu2g"` is `NULL`.
+
+By computing these results in independent CTEs and then joining on enrollment, we avoid referencing ax.enrollment directly in subqueries. 
+This approach is compatible with additional analytics engines and often provides better performance in PostgreSQL as well.
+
+### Overall flow
+
+Below is a high‐level overview of how the **`buildEnrollmentQueryWithCte`** method works and how it refactors the existing logic into a CTE‐based query. 
+
+Inside **`buildEnrollmentQueryWithCte(params)`**, the code proceeds in these key steps:
+
+1. **Collect CTE Definitions**
+    - It scans through the **`EventQueryParams`** (especially `params.getItems()`) to identify which columns need special handling. This includes:
+        - **Program Indicator** items
+        - **Items bound to a specific Program Stage** (sometimes with offsets, e.g., “nth event” logic)
+    - For each type of item, we build (or delegate building of) a CTE definition (SQL snippet). These definitions are stored in a **`CteContext`** object.
+
+2. **Generate CTE Filters**
+    - Additional filters may be needed for query items that have filters (`item.hasFilter()`). If such items require “latest event” logic or repeated‐stage handling, a separate **filter CTE** is generated.
+
+3. **Append the CTE Clause**
+    - We gather all generated CTE definitions from the **`CteContext`** and place them in a `WITH ...` clause at the start of the SQL.
+
+4. **Construct the Main SELECT**
+    - We pick the **standard enrollment columns** (e.g., `enrollmentdate`, `trackedentity`, `lastupdated`) plus any columns derived from **CTEs**.
+    - This step merges basic columns (from the main `analytics_enrollment_*` table) with additional “value” columns pulled out of the CTE definitions.
+
+5. **FROM and JOIN Logic**
+    - The **`FROM`** clause references the main enrollment analytics table (e.g. `FROM analytics_enrollment_m3xtlkyblki AS ax`).
+    - The code appends **LEFT JOIN** statements for each CTE that needs to link to the main table on `enrollment`.
+
+6. **WHERE Clause**
+    - The base “where” conditions come from the original logic (`getWhereClause(params)`), covering date ranges, organization units, statuses, etc.
+    - Additional filters are applied if they relate to columns computed in the CTEs (via `addCteFiltersToWhereClause`).
+
+7. **Sorting and Paging**
+    - Finally, an **ORDER BY** (if `params` is sorting on something) is appended.
+    - A **LIMIT/OFFSET** is added according to the required paging settings.
+
+When done, the **StringBuilder** contains the fully assembled SQL statement.
+
+## 3. Key Methods / Helpers
+
+Within **`buildEnrollmentQueryWithCte`**, you’ll see these helper calls:
+
+1. **`getCteDefinitions(params)`**
+    - Identifies all items (e.g., Program Stage columns, Program Indicators) that need a subquery.
+    - Creates a **CTE definition** for each item, handling “row_number” logic or Program Indicator subqueries.
+    - Stores definitions in a `CteContext`.
+
+2. **`generateFilterCTEs(params, cteContext)`**
+    - Looks for **filters** on items (e.g. `item.hasFilter()`).
+    - If an item needs a subquery filter (e.g. “the last event’s value must be X”), builds a “filter CTE” specifically for that.
+
+3. **`appendCteClause(sql, cteContext)`**
+    - Aggregates all CTE SQL fragments from the context.
+    - Organizes them in a `WITH cteName AS ( ... )` structure and appends to `sql`.
+
+4. **`appendSelectClause(sql, params, cteContext)`**
+    - Builds the `SELECT` part.
+    - Merges the default columns (e.g., `ax.enrollment`) with columns derived from each CTE definition (e.g. `cteAlias.value AS someColumn`).
+
+5. **`appendCteJoins(sql, cteContext)`**
+    - For each CTE, inserts a `LEFT JOIN cteName alias ON cteAlias.enrollment = ax.enrollment`.
+    - Enables referencing computed values in the main SELECT.
+
+6. **`appendWhereClause(sql, params, cteContext)`**
+    - Uses the original `getWhereClause(params)` for base filters.
+    - Plus merges in any filter conditions from the CTE context (e.g., `cteAlias.value = X`).
+
+7. **`appendSortingAndPaging(sql, params)`**
+    - Optionally appends an `ORDER BY ...`.
+    - Adds `LIMIT x OFFSET y` if relevant.
+
+## 4. Notable Changes from Pre‐CTE Logic
+
+1. **Elimination of Correlated Subqueries**
+    - Previously:
+      ```sql
+      SELECT
+        (SELECT ... FROM analytics_event_xxx WHERE enrollment = ax.enrollment ...)
+      ```
+      repeated for each column.
+    - Now replaced by a single (or multiple) CTE definitions, joined once.
+
+2. **Better Handling of Repeatable Stages**
+    - The offset logic (`createOffset(...)`) and row numbering (`row_number() OVER (...)`) are centralized in specialized CTEs instead of inline subqueries.
+
+3. **Filter Consolidation**
+    - Complex filters on “the most recent event for stage X” are turned into “filter CTEs” plus a straightforward check (e.g., `cteAlias.value = 'someFilter'`).

dhis-2/dhis-api/src/main/java/org/hisp/dhis/setting/SystemSettings.java

@@ -756,4 +756,8 @@ default String getPasswordValidationPattern() {
             "^(?=.*[A-Z])(?=.*[a-z])(?=.*\\d)(?=.*[\\W_])[A-Za-z\\d\\W_]{%d,%d}$",
             getMinPasswordLength(), getMaxPasswordLength()));
   }
+
+  default boolean getUseExperimentalAnalyticsQueryEngine() {
+    return asBoolean("experimentalAnalyticsSqlEngineEnabled", false);
+  }
 }

dhis-2/dhis-services/dhis-service-analytics/pom.xml

@@ -169,7 +169,10 @@
       <artifactId>lombok</artifactId>
       <scope>provided</scope>
     </dependency>
-
+    <dependency>
+      <groupId>com.github.jsqlparser</groupId>
+      <artifactId>jsqlparser</artifactId>
+    </dependency>
     <!-- Test -->
 
     <dependency>