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Oracle: a compiled Cypher statement kept in the driver's statement cache holds megabytes of PGA, and a few dozen run the instance out (ORA-04036)

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  • #221 von @jasperblues — gemerged

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Schwierigkeit
4/5
Geschätzter Aufwand
3-5 Tage
Anfängerfreundlichkeit
57/100
Issue-Typ
Bug
Klarheit
Klar beschrieben
Aktivitätsstatus
Aktiv
Tech-Stack
postgresql, python
Bereich
database, performance

Rechercherichtung

Start with drivine/connection/oracle/oracle_connection.py and drivine/sync/connection/oracle/oracle_connection.py, then check how OracleConnection.query and open_cursor prepare and execute Cypher statements. Port the described long-text memory test, measuring session PGA through v$sesstat as system; done means Cypher statements avoid the driver's statement cache and the test stays below 200 MB.

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Beschreibung

Port of liberation-data/drivine4j#313.

What happens here now. On Oracle, Oracle keeps a cursor's memory for as long as the driver keeps its statement in its statement cache. In drivine4j a compiled Cypher statement held about 4 MB of PGA while cached, 7 MB at 5,000 characters, and 10 to 24 MB when it read a text past 32 KB. A few dozen distinct statements took the instance from 300 MB to over 1.3 GB on one session. With more sessions that runs past Oracle Free's 2 GB PGA_AGGREGATE_LIMIT (ORA-04036). The memory is held on the server, not in the client, so any driver that caches the statement does the same.

OracleConnection.query and open_cursor (drivine/connection/oracle/oracle_connection.py:61 and :88, and drivine/sync/connection/oracle/oracle_connection.py:63 and :90) run a compiled Cypher statement with python-oracledb's default statement cache (stmtcachesize 20 per connection). The pool is set up in oracle_connection_provider.py with max=pool_max. So up to 20 compiled statements per session stay cached, each holding megabytes. This isn't measured here yet, but the cause is on the server side.

The fix in drivine4j (liberation-data/drivine4j#352): a statement compiled from Cypher isn't kept in the driver's cache on Oracle (statement.isPoolable = false), so it is closed when it ends. Drivine's own statements stay cached. The python-oracledb equivalent is cursor.prepare(statement, cache_statement=False) before execute, done only when the statement came from Cypher.

Tests to port.

  • OracleLongTextMemoryTest: 90 distinct long-text statements (given WITH $t AS t, stored MATCH (n) WITH n.t AS t, made WITH $a + $b AS t, each with left(t, size(t) - i)) on one session. It reads that session's session pga memory from v$sesstat as system and asserts it grew by less than 200 MB. In drivine4j it grew by 1,134 MB before the fix and by 37 MB after.
  • The long-text test now runs stored and made texts at 40,000 characters too, not only given ones.
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Python
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