In order to enable truncate for PostgreSQL databases in Spark JDBC, a change is needed to the query used for truncating a PostgreSQL table. By default, PostgreSQL will automatically truncate any descendant tables if a TRUNCATE query is executed. As this may result in (unwanted) side-effects, the query used for the truncate should be specified separately for PostgreSQL, specifying only to TRUNCATE a single table.
## What changes were proposed in this pull request?
Add `getTruncateQuery` function to `JdbcDialect.scala`, with default query. Overridden this function for PostgreSQL to only truncate a single table. Also sets `isCascadingTruncateTable` to false, as this will allow truncates for PostgreSQL.
## How was this patch tested?
Existing tests all pass. Added test for `getTruncateQuery`
Author: Daniel van der Ende <daniel.vanderende@gmail.com>
Closes#19911 from danielvdende/SPARK-22717.
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