Audience: business/tech readers. Short, straight.

1) Speed vs. Structure

• Redis: in‑memory key/value + simple data structures. Micro‑ to millisecond latency. Great when you read/write by key and need to go fast.
• SQL: durable relational store (PostgreSQL/MySQL, etc.). Rich queries (JOIN/GROUP BY), constraints, transactions. Designed for correctness and analysis.

Takeaway: If the question is “how fast can I fetch a known thing?”, Redis wins. If the question is “how do these things relate and what’s the total by X over time?”, SQL wins.

2) Ephemeral vs. System‑of‑Record

• Redis: data can be temporary. Keys can expire automatically using TTL (time‑to‑live); memory pressure can evict keys. It’s a speed layer.
• SQL: data is authoritative. Strong durability, backups, history, audits. It’s the system of record.

Takeaway: Redis handles hot/short‑lived data; SQL keeps the truth long‑term.

Can we keep data in Redis to avoid overloading the DB?

Yes—for the hot path. Store frequently accessed or pre‑computed results in Redis to offload your SQL:

• Cached rows/responses with a TTL (e.g., 5–15 min) → slashes read load on SQL.
• Pre‑aggregated dashboard tiles (top‑N, rolling counts) with TTL (e.g., 1–10 min) → fast dashboards without hammering SQL.
• Sessions/rate‑limits with TTL (minutes to hours) → no persistent footprint in SQL.

If Redis is flushed or a key expires, you recompute from SQL; users see a slower request, not data loss.

Quick decision

• Need blazing‑fast lookups or short‑lived counters? → Redis + TTL.
• Need history, joins, audits, ad‑hoc reporting? → SQL.
• Want both speed and correctness? → SQL as truth, Redis as cache with sensible TTLs.

Costs — quick reality check

Free SQL options

• PostgreSQL, MySQL, MariaDB, SQLite — no license fee. You still pay for compute, storage, backup, HA, and the people/time to operate it. Optional paid support from vendors (e.g., Postgres providers) if you want SLAs.

Paid SQL options

• Managed cloud SQL (e.g., RDS/Aurora, Cloud SQL/AlloyDB, Azure Database for PostgreSQL/MySQL): billed mainly by vCPU/RAM, storage GB‑month + IOPS/throughput, backup storage, and multi‑AZ. You’re paying to offload patching, backups, failover, and observability.
• Commercial/enterprise SQL (e.g., Oracle DB, Microsoft SQL Server Enterprise/Standard, enterprise Postgres distros): license/subscription on top of infra. You pay for features, support SLAs, and ecosystem tooling.

Redis pricing model

• Redis OSS — no license fee. Runs on your VMs/bare metal; you carry ops risk.
• Managed Redis (e.g., ElastiCache for Redis, Azure Cache for Redis, Redis Cloud/Enterprise): billed primarily by RAM size (GB‑hour), plus charges for replicas/shards, persistence/storage, network egress, and multi‑AZ/region. Memory is the dominant cost; each replica roughly doubles RAM.
• Persistence trade‑off: enabling AOF/RDB adds storage and can reduce peak throughput; good for recovery, but you still shouldn’t treat Redis as your system of record.

Cost strategy that works

• Keep SQL as truth; scale it thoughtfully (indexes, read replicas, materialized views).
• Use Redis as a pressure valve: cache the expensive reads and pre‑aggregates with TTL (e.g., 5–15 min for API rows, 1–10 min for dashboard tiles). TTL bounds memory usage and spend.
• Track cache hit rate and evictions; right‑size shards/replicas; avoid storing cold, long‑tail data in Redis.
• For reporting: SQL/warehouse holds history; Redis accelerates the hot slice only.

Bottom line

Use Redis to reduce pressure on your database by caching hot results with TTL. Keep the authoritative data in SQL for reporting and compliance.