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https://issues.apache.org/jira/browse/HIVE-4051?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13714281#comment-13714281
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Sergey Shelukhin commented on HIVE-4051:
----------------------------------------
I am actually getting 16 queries... 3 by PART_ID (itself, values, params), 10
per its SD_ID, 2 for SD_ID_OID (might also be SD_ID?), and 1 for SERDE.
Some normalization doesn't make any sense, e.g.
{code}
mysql> select name, slib, count(1) from SERDES;
+------+----------------------------------------------------+----------+
| name | slib | count(1) |
+------+----------------------------------------------------+----------+
| NULL | org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe | 32769 |
+------+----------------------------------------------------+----------+
{code}
if stored for each row 1-on-1 anyway, why not just store it in the row?
I am looking in the direction of doing a full-string query returning a bag of
fields, rather than MPartition, so that per-object calls are not done. Then SDs
can be fetched with join, etc.
One thing that is not quite clear is how to do fetch from separate tables for
which DN object doesn't exist e.g. PARTITION_KEY_VALS. Ideally we would fetch
all of these for all necessary partitions in one query and put them into
objects.
If all else fails we can use SQL
http://www.datanucleus.org/products/datanucleus/jdo/sql.html, but it will
halfway defeat the purpose of having DN in the first place, since it becomes
RDBMS-dependent.
In fact to avoid warring with datanucleus perhaps we could do SQL as
storage-specific optimization, configurable for metastore, and fall back to ORM
if disabled.
> Hive's metastore suffers from 1+N queries when querying partitions & is slow
> ----------------------------------------------------------------------------
>
> Key: HIVE-4051
> URL: https://issues.apache.org/jira/browse/HIVE-4051
> Project: Hive
> Issue Type: Bug
> Components: Clients, Metastore
> Environment: RHEL 6.3 / EC2 C1.XL
> Reporter: Gopal V
> Assignee: Sergey Shelukhin
>
> 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
> {code}
> 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)
> {code}
> 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.
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