For a read replica on a DB system,it must have at least 64 GB of memory(Answer D). This requirement ensures that the read replica has sufficient resources to handle read operations and maintain performance consistency with the primary DB system.
References:
* OCI MySQL Read Replicas Documentation

To connect to a MySQL DB system from an OCI Cloud Shell session when the VCN is not configured with VCN peering, VPN access, or FastConnect, the following must be true:
A:You must attach the Cloud Shell session to a VCN subnet that can connect to the MySQL DB system: The Cloud Shell must be in the same network or have a route to the network containing the MySQL DB system.
C:The MySQL DB system subnet must be in theCloud Shell tenancy home region: Cloud Shell sessions operate within the OCI home region, so the target MySQL DB system must be accessible from within the same region.

When performing a node estimate while adding a HeatWave cluster to a DB system in OCI Console, the process undertakes the following tasks:
* Estimate the number of OCPUs required to process selected user tables(Answer B): The node estimation process calculates the necessary OCPUs based on the size and complexity of the tables selected by the user. This ensures that the cluster has adequate processing power for efficient performance.
* Estimate the memory required for each user table(Answer C): This task involves determining the memory requirements for each table that will be loaded into the HeatWave cluster. This ensures that there is sufficient memory to handle the data and execute queries efficiently.
References:
* OCI Console Documentation on HeatWave
* MySQL HeatWave User Guide

Automatic DB system version upgrades in OCI are performed to the next release in sequence. This means that the system is upgraded to the next available version rather than skipping to the most recent release, ensuring a stable and tested upgrade path.

When creating a replication channel and selecting "Source cannot use GTID auto-positioning", it typically indicates that the source's GTID mode is off. GTID (Global Transaction Identifier) auto-positioning allows for automated recovery and positioning of transactions in replication, but it requires that GTID mode be enabled on the source database.