Details
Major Description
Hive's query client takes a long time to initialize & start planning queries because of delays in creating all the MTable/MPartition objects.
For a hive db with 1800 partitions, the metastore took 6-7 seconds to initialize - firing approximately 5900 queries to the mysql database.
Several of those queries fetch exactly one row to create a single object on the client.
The following 12 queries were repeated for each partition, generating a storm of SQL queries
4 Query SELECT `A0`.`SD_ID`,`B0`.`INPUT_FORMAT`,`B0`.`IS_COMPRESSED`,`B0`.`IS_STOREDASSUBDIRECTORIES`,`B0`.`LOCATION`,`B0`.`NUM_BUCKETS`,`B0`.`OUTPUT_FORMAT`,`B0`.`SD_ID` FROM `PARTITIONS` `A0` LEFT OUTER JOIN `SDS` `B0` ON `A0`.`SD_ID` = `B0`.`SD_ID` WHERE `A0`.`PART_ID` = 3945 4 Query SELECT `A0`.`CD_ID`,`B0`.`CD_ID` FROM `SDS` `A0` LEFT OUTER JOIN `CDS` `B0` ON `A0`.`CD_ID` = `B0`.`CD_ID` WHERE `A0`.`SD_ID` =4871 4 Query SELECT COUNT(*) FROM `COLUMNS_V2` THIS WHERE THIS.`CD_ID`=1546 AND THIS.`INTEGER_IDX`>=0 4 Query SELECT `A0`.`COMMENT`,`A0`.`COLUMN_NAME`,`A0`.`TYPE_NAME`,`A0`.`INTEGER_IDX` AS NUCORDER0 FROM `COLUMNS_V2` `A0` WHERE `A0`.`CD_ID` = 1546 AND `A0`.`INTEGER_IDX` >= 0 ORDER BY NUCORDER0 4 Query SELECT `A0`.`SERDE_ID`,`B0`.`NAME`,`B0`.`SLIB`,`B0`.`SERDE_ID` FROM `SDS` `A0` LEFT OUTER JOIN `SERDES` `B0` ON `A0`.`SERDE_ID` = `B0`.`SERDE_ID` WHERE `A0`.`SD_ID` =4871 4 Query SELECT COUNT(*) FROM `SORT_COLS` THIS WHERE THIS.`SD_ID`=4871 AND THIS.`INTEGER_IDX`>=0 4 Query SELECT `A0`.`COLUMN_NAME`,`A0`.`ORDER`,`A0`.`INTEGER_IDX` AS NUCORDER0 FROM `SORT_COLS` `A0` WHERE `A0`.`SD_ID` =4871 AND `A0`.`INTEGER_IDX` >= 0 ORDER BY NUCORDER0 4 Query SELECT COUNT(*) FROM `SKEWED_VALUES` THIS WHERE THIS.`SD_ID_OID`=4871 AND THIS.`INTEGER_IDX`>=0 4 Query SELECT 'org.apache.hadoop.hive.metastore.model.MStringList' AS NUCLEUS_TYPE,`A1`.`STRING_LIST_ID`,`A0`.`INTEGER_IDX` AS NUCORDER0 FROM `SKEWED_VALUES` `A0` INNER JOIN `SKEWED_STRING_LIST` `A1` ON `A0`.`STRING_LIST_ID_EID` = `A1`.`STRING_LIST_ID` WHERE `A0`.`SD_ID_OID` =4871 AND `A0`.`INTEGER_IDX` >= 0 ORDER BY NUCORDER0 4 Query SELECT COUNT(*) FROM `SKEWED_COL_VALUE_LOC_MAP` WHERE `SD_ID` =4871 AND `STRING_LIST_ID_KID` IS NOT NULL 4 Query SELECT 'org.apache.hadoop.hive.metastore.model.MStringList' AS NUCLEUS_TYPE,`A0`.`STRING_LIST_ID` FROM `SKEWED_STRING_LIST` `A0` INNER JOIN `SKEWED_COL_VALUE_LOC_MAP` `B0` ON `A0`.`STRING_LIST_ID` = `B0`.`STRING_LIST_ID_KID` WHERE `B0`.`SD_ID` =4871 4 Query SELECT `A0`.`STRING_LIST_ID_KID`,`A0`.`LOCATION` FROM `SKEWED_COL_VALUE_LOC_MAP` `A0` WHERE `A0`.`SD_ID` =4871 AND NOT (`A0`.`STRING_LIST_ID_KID` IS NULL)
This data is not detached or cached, so this operation is performed during every query plan for the partitions, even in the same hive client.
The queries are automatically generated by JDO/DataNucleus which makes it nearly impossible to rewrite it into a single denormalized join operation & process it locally.
Attempts to optimize this with JDO fetch-groups did not bear fruit in improving the query count.
