oleksiy-sayankin commented on a change in pull request #2323:
URL: https://github.com/apache/hive/pull/2323#discussion_r640590281
##########
File path:
standalone-metastore/metastore-server/src/main/java/org/apache/hadoop/hive/metastore/ObjectStore.java
##########
@@ -5218,31 +5218,55 @@ private void
removeUnusedColumnDescriptor(MColumnDescriptor oldCD) {
return;
}
- boolean success = false;
Query query = null;
+ LOG.debug("execute removeUnusedColumnDescriptor");
+ DatabaseProduct dbProduct =
DatabaseProduct.determineDatabaseProduct(MetaStoreDirectSql.getProductName(pm),
conf);
- try {
- openTransaction();
- LOG.debug("execute removeUnusedColumnDescriptor");
+ /**
+ * In order to workaround oracle not supporting limit statement caused
performance issue, HIVE-9447 makes
+ * all the backend DB run select count(1) from SDS where SDS.CD_ID=? to
check if the specific CD_ID is
+ * referenced in SDS table before drop a partition. This select count(1)
statement does not scale well in
+ * Postgres, and there is no index for CD_ID column in SDS table.
+ * For a SDS table with with 1.5 million rows, select count(1) has average
700ms without index, while in
+ * 10-20ms with index. But the statement before
+ * HIVE-9447( SELECT * FROM "SDS" "A0" WHERE "A0"."CD_ID" = $1 limit 1)
uses less than 10ms .
+ */
- query = pm.newQuery("select count(1) from " +
- "org.apache.hadoop.hive.metastore.model.MStorageDescriptor where
(this.cd == inCD)");
+ if (dbProduct.isPOSTGRES()) {
+ query = pm.newQuery(MStorageDescriptor.class, "this.cd == inCD");
query.declareParameters("MColumnDescriptor inCD");
- long count = ((Long)query.execute(oldCD)).longValue();
-
+ List<MStorageDescriptor> referencedSDs =
listStorageDescriptorsWithCD(oldCD, query);
Review comment:
I have found
[this](https://stackoverflow.com/questions/14667580/why-is-the-limit-clause-not-supported-in-sql-server).
They say
> MS SQL Server has TOP (n) clause. Example from SQL Server
> SELECT TOP 5 * FROM Table
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