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Qihan Zhang (张启涵)
A Brief Introduction
My name is Qihan Zhang, a CS PhD student at USC. I’m fortunate to be advised by Professor Ibrahim Sabek and to work closely with Mengyuan Li. I received my B.S. in Computer Science from UESTC (University of Electronic Science and Technology of China), where I was fortunate to be advised by Professor Hancong Duan. My research interests lie in database systems (especially query optimization), the intersection of AI and DB, and the intersection of security and DB.
The field of databases is a fascinating one: it reveals the elegance and complexity of system design while also offering glimpses of the everyday realities of software engineering. As AI continues to evolve rapidly, data serves as both fuel for models and sustenance for databases themselves. I believe this field demands deep expertise, yet remains open and full of possibilities.
The story begins on a summer afternoon, with the definitions of data, knowledge, and information. The young man drifts into a drowsy haze. He still believes that the essence of university lies in cultivating virtue, drawing close to the people, and reaching toward the ultimate good. Yet today, the rat race is almost everywhere, and inescapable. People hide behind ever more dazzling facades to cover up hearts grown weary and anxious. Slowly waking, he looks back and realizes how far he has come. Has he grown up? Matured? Or merely grown numb? Either way, there is no going back. —For those children who wander the streets alone in the darkest of nights, with nowhere to turn.
News
[Apr 2026] We have a new paper accepted by SeQureDB 2026, co-located with SIGMOD/PODS 2026! Running analytical databases inside confidential VMs (CVMs) such as AMD SEV-SNP incurs substantial overhead, yet prior work has largely benchmarked these slowdowns rather than optimized them. We trace the problem to a hardware–software mismatch — query optimizers still use KVM-oriented cost assumptions that no longer hold in CVMs — and propose a lightweight CVM-aware cost calibration extension that models data-movement and RMP-related translation overheads via simple physical proxies. Our calibration recovers up to 48% of the performance gap and even outperforms the non-encrypted baseline on some workloads.
[Oct 2025] We have a new paper accepted by GeoGenAgent 2025, which co-occurs with the SIGSpatial 2025! While Large Language Models (LLMs) exhibit latent geospatial knowledge, they suffer from inherent limitations in spatial understanding when tackling context-free topological questions. We exposed this limitation by testing LLMs across three prompting regimes (zero-shot, definitions, and chain-of-thought) and found that even guided reasoning exposes fundamental logical inconsistencies stemming from their core reasoning ability, not just a lack of external data.
[Aug 2025] We have a new paper (co-first author) accepted by SIGMOD 2026, which discuss the possibility of using RAG to optimize the query execution plan, the results show great generalizability.
[Jun 2025] We have a new paper (first author) accepted by VLDB 2025, which discuss the possibility of using lifelong reinforcement learning to optimize the query execution plan.
[May 2025] I will attend the SoCalDB 2025 and present a poster there!
[Apr 2025] We have a new workshop paper (co-first author) accepted by aiDM 2025, which co-occurs with the SIGMOD 2025!
[Mar 2025] I officially join the USC Viterbi School of Engineering as a CS PhD student!
The Myth of Sisyphus
Push the stone along the fixed, irregular and steep mountain road using your mouse or control panel. Once you reach the top, watch it roll back down along the same path to the start, endlessly.
But one must imagine Sisyphus happy. —Albert Camus
The Iron House Metaphor
Imagine an iron house, completely sealed, without any windows or doors.
Inside it, people are sleeping, unaware of their approaching suffocation and death.
They will pass from sleep into death without suffering or sorrow, simply drifting away.
You stand here, conscious, facing this scene. You have two choices:
I hope that Chinese youth can shed negativity and focus on moving forward, without needing to listen to those who surrender and self-destruct. Let those who can take action do so, and let those who can speak voice their thoughts. Even with a spark of passion, shine like fireflies; you can also emit a little light in the darkness without waiting for a torch. —Xun Lu
The Perpetually Young at Heart
Far and Near
You, At times look at me, At times look at the clouds. I feel, When you look at me, you are far. When you look at the clouds, you are near.
I Am a Willful Child
I am a willful child— I want to paint windows everywhere on the earth, So that all eyes long used to darkness May learn to grow accustomed to light.
Perhaps I am a child spoiled by my mother— I am headstrong, I hope Every moment May glow with the beauty of colored crayons. I hope To draw upon my beloved white pages, To sketch a clumsy kind of freedom, To paint an eye that will never cry, A piece of sky, A feather and leaf that belong to that sky, A pale green evening, and an apple.
I want to paint the morning, To paint a smile The dew can behold, To paint the youngest loves That know no sorrow, To paint my imagined beloved— She has never seen dark clouds, Her eyes are the color of a clear, bright sky, She always watches me, Always watches, Never suddenly turning away.
I want to paint distant landscapes, To paint a clean horizon and trembling ripples, To paint countless happy little rivers, To paint hills Soft with pale fuzz. I will set them close together, Let them fall in love, Let every quiet consent, Every silent thrill of spring, Become a birthday bloom.
I also want to paint the future— I have never seen her, nor can I, Yet I know she is beautiful. I will paint her autumn coat, Paint the burning candles and maple leaves, Paint all those hearts snuffed out For love of her. I will paint a wedding, Paint each holiday that wakes too early— Adorned with glossy candy wrappers And illustrations from a northern fairy tale.
I am a willful child, I want to wipe away all misfortune, I want to fill the earth with windows, So all eyes accustomed to darkness May learn to love the light.
I want to paint the wind, To paint ranges of mountains ever higher, To paint the longing of eastern peoples, To paint the sea— Boundless, brimming with joyful sound.
At last, in a corner of the page, I still want to paint myself— To paint a koala, Sitting in Victoria’s dusky forests, Quiet atop a branch, Lost in thought. He has no home, No distant heart, Only a bounty of berry-like dreams, And very large, very gentle eyes.
I am hoping, I am imagining, But I do not know why I have never been given crayons, Never gained a single colored moment. I have only myself, My fingers, and my wounds. I must tear those beloved blank pages, Let them search for butterflies, Let them vanish from today.
I am a child, A child spoiled by the mother of dreams. I am willful. ——Cheng Gu, one of Chinese Misty Poets.