postgresql - Exposed with DataSource を Postgres で使用する場合、インデックスは使用されません

Question

Exposed を DataSource で使用すると、予期しない動作に直面しています (Apache DBCP と HikariCP を試しました)。

セットアップ: 単一のテーブル ( test) とid、flagのインデックスを持つフィールドflag。

クエリ:SELECT * from test where flag=1 limit 1;

手動で実行すると、インデックスが使用され、クエリが高速になります。公開されたものを介して繰り返し実行すると、9 回の呼び出しの後、パフォーマンスが低下します。インデックスはもう使用されていません。以下のクエリプランを参照してください。

コード例は次のとおりです。


object TestTable : IntIdTable() {
    val flag = integer("flag").index()
}

fun createNRows(n: Int) = repeat(n) {
    TestTable.insert { it[flag] = 0 }
}

fun main(args: Array<String>) {
    val ds = HikariDataSource(HikariConfig().apply {
        jdbcUrl = "jdbc:postgresql://localhost:5432/testdb"
        username = ...
        password = ...
        setDriverClassName("org.postgresql.Driver")
    })

    Database.connect(ds)


    transaction {
        // only run the first time:
        // SchemaUtils.create(TestTable)
        // createNRows(1000000) 
        println("total ${TestTable.selectAll().count()} elements")
    }

    repeat(10000) {
        transaction {
            val startedAt = System.currentTimeMillis()
            TestTable.select { TestTable.flag.eq(1) }.limit(1).toList()
            println("Query took ${System.currentTimeMillis() - startedAt}")
        }
    }
}

出力：

total 1000000 elements
Query took 6
Query took 1
Query took 1
Query took 1
Query took 1
Query took 1
Query took 1
Query took 1
Query took 0
Query took 79
Query took 64
Query took 63
Query took 62
Query took 63
....

EXPLAIN (ANALYZE, BUFFERS)有効にしたpostgresログは次のとおりです。

これは最後の高速クエリです。

2020-03-10 23:03:00.596 CET [71012] LOG:  duration: 0.021 ms  bind S_2: 
2020-03-10 23:03:00.597 CET [71012] LOG:  duration: 0.083 ms  parse <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL REPEATABLE READ
2020-03-10 23:03:00.597 CET [71012] LOG:  duration: 0.013 ms  bind <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL REPEATABLE READ
2020-03-10 23:03:00.597 CET [71012] LOG:  execute <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL REPEATABLE READ
2020-03-10 23:03:00.597 CET [71012] LOG:  duration: 0.025 ms
2020-03-10 23:03:00.597 CET [71012] LOG:  duration: 0.011 ms  bind S_3: BEGIN
2020-03-10 23:03:00.597 CET [71012] LOG:  execute S_3: BEGIN
2020-03-10 23:03:00.597 CET [71012] LOG:  duration: 0.015 ms
2020-03-10 23:03:00.598 CET [71012] LOG:  duration: 0.159 ms  bind S_4: SELECT test.id, test.flag FROM test WHERE test.flag = $1 LIMIT 1
2020-03-10 23:03:00.598 CET [71012] DETAIL:  parameters: $1 = '1'
2020-03-10 23:03:00.598 CET [71012] LOG:  execute S_4: SELECT test.id, test.flag FROM test WHERE test.flag = $1 LIMIT 1
2020-03-10 23:03:00.598 CET [71012] DETAIL:  parameters: $1 = '1'
2020-03-10 23:03:00.598 CET [71012] LOG:  duration: 0.028 ms
2020-03-10 23:03:00.598 CET [71012] LOG:  duration: 0.015 ms  plan:
    Query Text: SELECT test.id, test.flag FROM test WHERE test.flag = $1 LIMIT 1
    Limit  (cost=0.42..4.44 rows=1 width=8) (actual time=0.013..0.013 rows=0 loops=1)
      Buffers: shared hit=3
      ->  Index Scan using test_flag on test  (cost=0.42..4.44 rows=1 width=8) (actual time=0.012..0.012 rows=0 loops=1)
            Index Cond: (flag = 1)
            Buffers: shared hit=3
2020-03-10 23:03:00.598 CET [71012] LOG:  duration: 0.072 ms  bind S_1: COMMIT
2020-03-10 23:03:00.598 CET [71012] LOG:  execute S_1: COMMIT
2020-03-10 23:03:00.598 CET [71012] LOG:  duration: 0.017 ms
2020-03-10 23:03:00.599 CET [71012] LOG:  duration: 0.022 ms  parse <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL READ COMMITTED
2020-03-10 23:03:00.599 CET [71012] LOG:  duration: 0.007 ms  bind <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL READ COMMITTED
2020-03-10 23:03:00.599 CET [71012] LOG:  execute <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL READ COMMITTED
2020-03-10 23:03:00.599 CET [71012] LOG:  duration: 0.013 ms

そして、これが最初の「遅い」ものです。

2020-03-10 23:03:01.601 CET [71012] LOG:  duration: 0.022 ms  bind S_2: 
2020-03-10 23:03:01.602 CET [71012] LOG:  duration: 0.052 ms  parse <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL REPEATABLE READ
2020-03-10 23:03:01.602 CET [71012] LOG:  duration: 0.011 ms  bind <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL REPEATABLE READ
2020-03-10 23:03:01.602 CET [71012] LOG:  execute <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL REPEATABLE READ
2020-03-10 23:03:01.602 CET [71012] LOG:  duration: 0.023 ms
2020-03-10 23:03:01.602 CET [71012] LOG:  duration: 0.012 ms  bind S_3: BEGIN
2020-03-10 23:03:01.602 CET [71012] LOG:  execute S_3: BEGIN
2020-03-10 23:03:01.602 CET [71012] LOG:  duration: 0.015 ms
2020-03-10 23:03:01.602 CET [71012] LOG:  duration: 0.192 ms  bind S_4: SELECT test.id, test.flag FROM test WHERE test.flag = $1 LIMIT 1
2020-03-10 23:03:01.602 CET [71012] DETAIL:  parameters: $1 = '1'
2020-03-10 23:03:01.602 CET [71012] LOG:  execute S_4: SELECT test.id, test.flag FROM test WHERE test.flag = $1 LIMIT 1
2020-03-10 23:03:01.602 CET [71012] DETAIL:  parameters: $1 = '1'
2020-03-10 23:03:01.678 CET [71012] LOG:  duration: 75.889 ms
2020-03-10 23:03:01.679 CET [71012] LOG:  duration: 75.868 ms  plan:
    Query Text: SELECT test.id, test.flag FROM test WHERE test.flag = $1 LIMIT 1
    Limit  (cost=0.00..0.02 rows=1 width=8) (actual time=75.864..75.864 rows=0 loops=1)
      Buffers: shared hit=96 read=4329
      ->  Seq Scan on test  (cost=0.00..16925.00 rows=1000000 width=8) (actual time=75.862..75.862 rows=0 loops=1)
            Filter: (flag = $1)
            Rows Removed by Filter: 1000000
            Buffers: shared hit=96 read=4329
2020-03-10 23:03:01.679 CET [71012] LOG:  duration: 0.054 ms  bind S_1: COMMIT
2020-03-10 23:03:01.679 CET [71012] LOG:  execute S_1: COMMIT
2020-03-10 23:03:01.679 CET [71012] LOG:  duration: 0.014 ms
2020-03-10 23:03:01.679 CET [71012] LOG:  duration: 0.025 ms  parse <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL READ COMMITTED
2020-03-10 23:03:01.679 CET [71012] LOG:  duration: 0.004 ms  bind <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL READ COMMITTED
2020-03-10 23:03:01.679 CET [71012] LOG:  execute <unnamed>: SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL READ COMMITTED
2020-03-10 23:03:01.679 CET [71012] LOG:  duration: 0.009 ms

Postgres バージョン (自作):

postgres (PostgreSQL) 11.5

クライアントのバージョン:


dependencies {
    implementation 'org.jetbrains.exposed:exposed:0.17.7'
    implementation "org.postgresql:postgresql:42.2.8"
    implementation 'org.jetbrains.kotlin:kotlin-stdlib'
    implementation 'com.zaxxer:HikariCP:2.3.2'
}

postgres 構成がデフォルトです (ログは auto-explain で生成されますが、問題はそれなしで再現されます)

例のソースは次のとおりです: https://github.com/RomanBrodetski/kotlin-exposed-issue

所見:

が削除された場合.limit(1)、問題は再現されません
Database.connect("jdbc:postgresql://localhost:5432/testdb", driver = "org.postgresql.Driver")(の代わりに)データソースが使用されていない場合Database.connect(ds)、問題は再現されません。
トランザクションに追加のステートメントがある場合、問題は再現されません。

postgresql - Exposed with DataSource を Postgres で使用する場合、インデックスは使用されません

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