Mutation-based fuzzing is one of the most effective techniques for uncovering bugs in Database Management Systems (DBMSs). However, its effectiveness critically depends on the quality of the initial seed queries. High-quality seeds should be syntactically and semantically valid, incorporate diverse SQL features, and encode behaviors that drive execution into bug-prone states. In practice, existing DBMS fuzzers primarily rely on SQL queries extracted from unit tests or regression suites as initial seeds, which are often limited in diversity and scale, leaving many DBMS features and execution paths unexplored. To address this limitation, we propose SmartFuzz, an automated framework for synthesizing high-quality initial SQL seeds for mutation-based DBMS fuzzing using Large Language Models (LLMs). The key insight behind SmartFuzz is that two underutilized sources—official DBMS documentation and historical crash-triggering inputs—capture complementary knowledge about DBMS feature usage and bug-relevant behaviors. SmartFuzz extracts structured features from these sources and leverages LLMs to synthesize executable, feature-rich SQL seeds that are biased toward bug-prone execution states. We integrate SmartFuzz into existing mutation-based DBMS fuzzing pipelines and evaluate it on 4 widely used DBMSs. The results demonstrate that SmartFuzz significantly improves bug discovery and code coverage compared to state-of-the-art mutation-based fuzzers. In total, SmartFuzz detects 61 previously unknown bugs, of which 53 have been confirmed and fixed by developers.