# Parameters in RLWE

Let $$n, q, \sigma$$ be the polynomial degree($$x^n+1$$), coefficient modulo, and the standard derivation, respectively. I often see some parameters such as

For RLWE, we can use the CRT to decompose the $$\text{RLWE}_{q}$$ to some $$\text{RLWE}_{q_i}$$ for $$1\leq i\leq l$$, where $$q = q_1 q_2\cdots q_l$$, then when we consider the security of RLWE, we should take $$\log q$$ or $$\log q_i$$ to be considered?

You should consider $$\log q$$.
Considering that all the other parameters are fixed, the smaller $$q$$ is, the higher is the security. Even the table in your question is showing this (the table probably supposes that $$\sigma$$ is a small fixed value).
The hardness of the LWE and RLWE problems increases as the ratio $$q / || \text{noise}||$$ decreases, i.e., larger noise for the same $$q$$ makes the (R)LWE harder. You can think about the two extreme cases: if the noise is zero, then you can find the secret $$s$$ with Gaussian elimination; if the noise is as big as $$q$$, then it is essentially impossible to find $$s$$ because everything will be (very close to) uniform.
So if you take your (R)LWE samples $$(a, b)$$ defined mod $$q$$ and reduce mod a smaller $$q'$$ (that divides $$q$$), your new samples have the same noise terms, but with respect to a smaller modulus, so the ratio $$q' / || \text{noise}||$$ is smaller and the (R)LWE instance that you get is harder.