ORA-00093: pga_aggregate_limit must be between...

Starting 18c, pga_aggregate_limit setting has also been made dependent on PROCESSES initialization parameter. You might face ORA-00093 during startup of instance if value of pga_aggregate_limit is below the required value. For my 19c instance I faced same issue as follows.

SQL> startup nomount ORA-00093: pga_aggregate_limit must be between 45000M and 100000G ORA-01078: failure in processing system parameters SQL>

Direct Path Read and Full Table Scan

 Before reading this article, you might want to read this article about poor buffer cache hit ratio because of full table scans. In this article I will explain a feature of Oracle introduced after 10 to avoid flooding of database buffer cache because of full table scans. Database block is the unit of IO in Oracle, which means that whenever some data is read from the disks, one copy of data block is put in the buffer cache so that this data can be reused later to avoid reading same block from the disk in future. After that, the copy of data is used for user’s SQL.