What are not composed of sga in oracle database

Oracle SGA is part of the memory area of ​​the database instance, used to cache data and control information to improve performance. It contains buffer cache, redo log cache, shared pool, and Java pool, but does not contain user session-related memory (PGA), operating system kernel memory, database files, and non-database-related memory. A deep understanding of the composition and missing content of SGA is crucial for database performance tuning.

Oracle SGA: What exactly is it and what is missing?

You may have heard the word "SGA" (System Global Area) while learning Oracle databases. But what exactly is it? More importantly, it has nothing? This is the key to understanding SGA. Many beginners only know that SGA is part of memory and is used to cache data, but this is far from enough. We have to dig deeper to truly master it.

The goal of this article is to take you into the understanding of the composition of Oracle SGA and what it does not contain . After reading, you will have a clearer and more comprehensive understanding of SGA and better understand the performance tuning of the database.

First of all, we have to be clear: SGA is not the entire memory of Oracle database. It is just a part of the memory area of ​​the database instance, used to cache data and control information to improve database performance. Other memory parts of the database instance, such as the program global area (PGA, Program Global Area), are not within the scope of SGA.

So, what components does SGA contain? Classic SGA structures include buffer cache (Database Buffer Cache), redo log cache (Redo Log Buffer), shared pool (Shared Pool), Java Pool (Java Pool), etc.

Let's take a closer look at these components:

• Database Buffer Cache: This is the most important part of SGA, used to cache database data blocks. When the database needs to read data, it will first look up in the buffer cache. If found, it will be read directly from the cache, which is extremely fast; if not found, it will be read from disk, which is relatively slow. The cache hit rate is a key metric for measuring database performance and directly affects the database I/O performance. There are many optimization techniques here, such as adjusting the cache size, choosing the right cache replacement algorithm, etc., which are all advanced topics.
• Redo Log Buffer: This cache is used to store redo logs for database transactions. Redo logs are the key to database recovery, ensuring that the database can be restored to a consistent state after failure. After this cache is full, it will be written to the redo log file. Its design is very clever, ensuring the persistence of transactions, but the size needs to be carefully selected. Too small may lead to frequent writes to disk, affecting performance; too large will waste memory.
• Shared Pool: This area caches the database's shared SQL statements, PL/SQL code, data dictionary information, etc. When the database executes SQL statements, it first looks in the shared pool. If found, use it directly to avoid repeated parsing and compilation, thereby improving database performance. The management of shared pools is relatively complex, involving LRU (Least Recently Used) algorithms, etc. Optimizing shared pools requires in-depth understanding of these algorithms.
• Java Pool: As the name suggests, this is used to store Java-related resources. If you use Java programs in your database, this pool comes in handy.

Now, let's go back to the topic of the article: What does SGA contain ?

SGA does not contain memory related to user sessions. These memory belongs to PGAs, and each user session has its own PGA. The PGA stores session-specific information, such as the execution plan of SQL statements, sorting areas, etc. Confusing SGA and PGA is a common mistake for beginners. Understanding the difference between the two is crucial to tuning database performance.

SGA does not contain the memory of the operating system kernel, the database file itself, and other non-database-related memory areas.

In short, the key to understanding SGA is not only about understanding its composition, but also about understanding what it does not contain . Only in this way can we make more accurate judgments when tuning database performance and avoid detours. Remember, SGA is only part of the memory of the database instance. It works in conjunction with other memory areas to ensure the stability and high performance of the database. Take a deeper look at SGA and you will find this is a challenging and charismatic area.

Finally, a simple Python script is attached to simulate the simple working principle of buffer cache in SGA (for reference only, not a real Oracle SGA implementation):

 <code class="python">class BufferCache: def __init__(self, size): self.size = size self.cache = {} # 模拟缓存，用字典表示self.lru = [] # 模拟LRU列表def get(self, key): if key in self.cache: self.lru.remove(key) # 提升到列表头部self.lru.insert(0, key) return self.cache[key] return None def put(self, key, value): if len(self.cache) >= self.size: # 缓存已满，移除LRU列表尾部的元素evicted_key = self.lru.pop() del self.cache[evicted_key] self.cache[key] = value self.lru.insert(0, key) # 示例cache = BufferCache(3) cache.put("A", 10) cache.put("B", 20) cache.put("C", 30) print(cache.get("B")) # 输出20 cache.put("D", 40) # 缓存已满，"A" 被移除print(cache.get("A")) # 输出None</code>

Remember, this is just a simplified model, and the real Oracle buffer cache is much more complex than that.

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