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feat: EXPOSED-560 Support DISTINCT ON from Postgres #2275

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Oct 30, 2024
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feat: EXPOSED-560 Support DISTINCT ON from Postgres #2275

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7a67160
feat: EXPOSED-560 Support DISTINCT ON from Postgres
obabichevjb Oct 15, 2024
667fe1d
Update exposed-core/src/main/kotlin/org/jetbrains/exposed/sql/Query.kt
obabichevjb Oct 15, 2024
7cc0529
Review issues: prevent using both withDistinct and withDistinctOn, fi…
obabichevjb Oct 17, 2024
4f41be1
Review issues: add test with empty distinctOn, update query so it doe…
obabichevjb Oct 29, 2024
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19 changes: 18 additions & 1 deletion documentation-website/Writerside/topics/DSL-CRUD-operations.topic
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Expand Up @@ -171,11 +171,28 @@

<code-block lang="kotlin">
val directors =
StarWarsFilms.select(StarWarsFilms.director).where { StarWarsFilms.sequelId less 5 }.withDistinct()
StarWarsFilms.select(StarWarsFilms.director)
.where { StarWarsFilms.sequelId less 5 }.withDistinct()
.map {
it[StarWarsFilms.director]
}
</code-block>

<p>Some SQL dialects, such as PostgreSQL and H2, also support the <code>DISTINCT ON</code> clause.
You can use this clause with the <code>withDistinctOn()</code> function:</p>

<code-block lang="kotlin">
val directors =
StarWarsFilms.select(StarWarsFilms.director, StarWarsFilms.name)
.withDistinctOn(StarWarsFilms.director)
.orderBy(
StarWarsFilms.director to SortOrder.ASC,
StarWarsFilms.name to SortOrder.ASC
)
.map {
it[StarWarsFilms.name]
}
</code-block>
</chapter>
<chapter id="selectAll">
<title><code>selectAll</code></title>
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4 changes: 4 additions & 0 deletions exposed-core/api/exposed-core.api
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Expand Up @@ -1905,6 +1905,7 @@ public class org/jetbrains/exposed/sql/Query : org/jetbrains/exposed/sql/Abstrac
public synthetic fun forUpdate (Lorg/jetbrains/exposed/sql/vendors/ForUpdateOption;)Lorg/jetbrains/exposed/sql/SizedIterable;
public final fun getComments ()Ljava/util/Map;
public final fun getDistinct ()Z
public final fun getDistinctOn ()Ljava/util/List;
public final fun getGroupedByColumns ()Ljava/util/List;
public final fun getHaving ()Lorg/jetbrains/exposed/sql/Op;
protected fun getQueryToExecute ()Lorg/jetbrains/exposed/sql/statements/Statement;
Expand All @@ -1918,11 +1919,14 @@ public class org/jetbrains/exposed/sql/Query : org/jetbrains/exposed/sql/Abstrac
public synthetic fun notForUpdate ()Lorg/jetbrains/exposed/sql/SizedIterable;
public fun prepareSQL (Lorg/jetbrains/exposed/sql/QueryBuilder;)Ljava/lang/String;
protected final fun setDistinct (Z)V
protected final fun setDistinctOn (Ljava/util/List;)V
public fun setSet (Lorg/jetbrains/exposed/sql/FieldSet;)V
public final fun where (Lkotlin/jvm/functions/Function1;)Lorg/jetbrains/exposed/sql/Query;
public final fun where (Lorg/jetbrains/exposed/sql/Op;)Lorg/jetbrains/exposed/sql/Query;
public synthetic fun withDistinct (Z)Lorg/jetbrains/exposed/sql/AbstractQuery;
public fun withDistinct (Z)Lorg/jetbrains/exposed/sql/Query;
public final fun withDistinctOn ([Lkotlin/Pair;)Lorg/jetbrains/exposed/sql/Query;
public final fun withDistinctOn ([Lorg/jetbrains/exposed/sql/Column;)Lorg/jetbrains/exposed/sql/Query;
}

public final class org/jetbrains/exposed/sql/Query$CommentPosition : java/lang/Enum {
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41 changes: 41 additions & 0 deletions exposed-core/src/main/kotlin/org/jetbrains/exposed/sql/Query.kt
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Expand Up @@ -24,6 +24,15 @@ open class Query(override var set: FieldSet, where: Op<Boolean>?) : AbstractQuer
var distinct: Boolean = false
protected set

/**
* List of columns on which the query should be distinct.
*
* This parameter specifies columns for the `DISTINCT ON` clause, which allows selecting distinct rows based on
* the specified columns and is supported by some SQL dialects (e.g., PostgreSQL, H2).
*/
var distinctOn: List<Column<*>>? = null
protected set

/** The stored list of columns for a `GROUP BY` clause in this `SELECT` query. */
var groupedByColumns: List<Expression<*>> = mutableListOf()
private set
Expand Down Expand Up @@ -80,6 +89,35 @@ open class Query(override var set: FieldSet, where: Op<Boolean>?) : AbstractQuer
distinct = value
}

/**
* Specifies that the `SELECT` query should retrieve distinct results based on the given list of columns.
*
* This method can be used to set a `DISTINCT ON` clause for the query, which is supported by some SQL dialects
* (e.g., PostgreSQL, H2). The resulting query will retrieve rows that are distinct based on the specified columns.
*
* @param columns The columns to apply the `DISTINCT ON` clause.
* @return The current `Query` instance with the `DISTINCT ON` clause applied.
*/
fun withDistinctOn(vararg columns: Column<*>): Query = apply {
distinctOn = (distinctOn ?: emptyList()) + columns
}

/**
* Specifies that the `SELECT` query should retrieve distinct results based on the given list of columns with sort orders.
* This method sets a `DISTINCT ON` clause and may reorder the results as indicated.
*
* This method can be used to set a `DISTINCT ON` clause for the query, which is supported by some SQL dialects
* (e.g., PostgreSQL, H2), along with an `ORDER BY` clause for the specified columns.
*
* @param columns The columns and their sort orders to apply the `DISTINCT ON` clause.
* @return The current `Query` instance with the `DISTINCT ON` clause and reordering applied.
*/
fun withDistinctOn(vararg columns: Pair<Column<*>, SortOrder>): Query = apply {
@Suppress("SpreadOperator")
withDistinctOn(*columns.map { it.first }.toTypedArray())
return orderBy(*columns)
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(minor) with this implementation it would currently be allowed to -- arguably unintentionally -- add an entry to orderByExpressions twice for a DISTINCT ON column (since that attribute is a list and not a distinct set), e.g.

table
  .selectAll()
  .withDistinctOn(table.col to SortOrder.ASC)
  .orderBy(table.col, SortOrder.DESC)

postgresql allows for this and uses the last specified sorting predicate specified for a column but i'm not sure if you're intentionally supporting this use case; i don't personally know if the behaviour of having a column mentioned twice in ORDER BY is consistent across all sql dialects.

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Thank you for the feedback,

when I was adding the option of withDistinctOn with ordering I also thought that it could cause misunderstanding,

In general, the main purpose of that variant is make a use case when there is only distinctOn without extra ordering simpler, and prevent necessity to call orderBy right after withDistinctOn() with the same columns order, what could be broken on any refactoring.

So my expectation at the current moment that for any complex logic of adding ordering, the version withDistinctOn(vararg columns: Column<*>) (without ordering) should be used.

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note that if withDistinctOn() is called after orderBy() (which i think is technically possible) then the ordering may be incorrect (DISTINCT ON at least in postgresql requires the specified columns to appear in the ORDER BY expression in the same order), e.g.

table
  .selectAll()
  .orderBy(table.colThree)
  .withDistinctOn(table.colOne, table.colTwo)

i think results in ORDER BY table.col_three, table.col_one, table.col_two which generates a SQL error.

a possible solution would be to manipulate the underlying orderByExpressions mutable list directly to prepend the list of columns passed in, instead of calling orderBy(). something like:

columns.forEachIndexed { index, column -> (orderByExpressions as MutableList).add(index, column) }

(since private set on the declaration orderByExpressions prohibits redefining the var)

}

@Deprecated(
message = "As part of SELECT DSL design changes, this will be removed in future releases.",
replaceWith = ReplaceWith("adjustSelect { body.invoke() }"),
Expand Down Expand Up @@ -174,6 +212,9 @@ open class Query(override var set: FieldSet, where: Op<Boolean>?) : AbstractQuer
if (distinct) {
append("DISTINCT ")
}
distinctOn?.let { columns ->
columns.appendTo(prefix = "DISTINCT ON (", postfix = ")") { +"${it.table.tableName}.${it.name}" }
}
set.realFields.appendTo { +it }
}
if (set.source != Table.Dual || currentDialect.supportsDualTableConcept) {
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57 changes: 57 additions & 0 deletions exposed-tests/src/test/kotlin/org/jetbrains/exposed/sql/tests/shared/dml/DistinctOnTests.kt
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@@ -0,0 +1,57 @@
package org.jetbrains.exposed.sql.tests.shared.dml

import org.jetbrains.exposed.dao.id.IntIdTable
import org.jetbrains.exposed.sql.SortOrder
import org.jetbrains.exposed.sql.batchInsert
import org.jetbrains.exposed.sql.selectAll
import org.jetbrains.exposed.sql.tests.DatabaseTestsBase
import org.jetbrains.exposed.sql.tests.TestDB
import org.jetbrains.exposed.sql.tests.shared.assertEqualLists
import org.junit.Test

class DistinctOnTests : DatabaseTestsBase() {
@Test
fun testDistinctOn() {
val tester = object : IntIdTable("distinct_function_test") {
val value1 = integer("value1")
val value2 = integer("value2")
}

withTables(excludeSettings = TestDB.ALL - TestDB.ALL_POSTGRES - TestDB.ALL_H2, tester) {
tester.batchInsert(
listOf(
listOf(1, 1), listOf(1, 2), listOf(1, 2),
listOf(2, 1), listOf(2, 2), listOf(2, 2),
listOf(4, 4), listOf(4, 4), listOf(4, 4),
)
) {
this[tester.value1] = it[0]
this[tester.value2] = it[1]
}

val distinctValue1 = tester.selectAll()
.withDistinctOn(tester.value1)
.orderBy(tester.value1 to SortOrder.ASC, tester.value2 to SortOrder.ASC)
.map { it[tester.value1] to it[tester.value2] }
assertEqualLists(listOf(1 to 1, 2 to 1, 4 to 4), distinctValue1)

val distinctValue2 = tester.selectAll()
.withDistinctOn(tester.value2)
.orderBy(tester.value2 to SortOrder.ASC, tester.value1 to SortOrder.ASC)
.map { it[tester.value1] to it[tester.value2] }
assertEqualLists(listOf(1 to 1, 1 to 2, 4 to 4), distinctValue2)

val distinctBoth = tester.selectAll()
.withDistinctOn(tester.value1, tester.value2)
.orderBy(tester.value1 to SortOrder.ASC, tester.value2 to SortOrder.ASC)
.map { it[tester.value1] to it[tester.value2] }
assertEqualLists(listOf(1 to 1, 1 to 2, 2 to 1, 2 to 2, 4 to 4), distinctBoth)

val distinctSequential = tester.selectAll()
.withDistinctOn(tester.value1 to SortOrder.ASC)
.withDistinctOn(tester.value2 to SortOrder.ASC)
.map { it[tester.value1] to it[tester.value2] }
assertEqualLists(distinctBoth, distinctSequential)
}
}
}
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