[Bug]: FmtCtx.WriteValue quotes string literals with Go %q syntax — re-serialized SQL breaks or drifts under NO_BACKSLASH_ESCAPES / ANSI_QUOTES

[fengttt]

Summary

FmtCtx.WriteValue (pkg/sql/parsers/tree/format.go) formats P_char values under quoteString with Go's %q:

case P_char:
    return ctx.WriteString(fmt.Sprintf("%q", v))

%q produces a Go-syntax double-quoted string, not a MySQL string literal. Any statement re-serialized through WithQuoteString(true) and later re-parsed (CTAS follow-up INSERT ... SELECT, index/binder SQL, authenticate paths — see callers below) gets literals that are wrong or invalid under supported sql modes.

This is the generalization of the MATCH-pattern case fixed in #25683: that fix made FullTextMatchExpr mode-aware, but every other string literal still goes through the mode-blind %q path.

Verified failure classes (probe on current main)

Parsed each input under default mode, formatted with WithQuoteString(true), re-parsed under various modes:

1. Backslash doubling under NO_BACKSLASH_ESCAPES — value x\y formats as "x\\y"; NBE re-parse yields literal x\\y (backslashes doubled, silent value drift — same bug class as [Bug] CREATE TABLE AS SELECT with MATCH...AGAINST drops string-literal quotes during rewrite #24823 but for ordinary literals).

2. Double quote in value → invalid SQL under NO_BACKSLASH_ESCAPES and ANSI_QUOTES — value say "hi" formats as "say \"hi\"":

   • NBE re-parse: syntax error (backslash is not an escape, the \" terminates the string).
   • ANSI_QUOTES re-parse: syntax error (double quotes delimit identifiers). A CTAS whose source contains such a literal fails outright under these session modes.

3. Control characters silently change value under NBE — value containing a TAB formats as "tab\there"; NBE re-parse yields literal backslash-t (tab\there), not a TAB.

Additionally (unverified but follows from %q semantics): non-printable bytes are emitted as Go escapes (\x01, é) that MySQL's escaping rules parse differently (\x is not a MySQL escape).

Note ANSI_QUOTES breaks even in the default-session case whenever the session sets it: any %q-quoted literal becomes an identifier reference.

Affected production callers of WithQuoteString(true)

• pkg/sql/plan/build_ddl.go:1385,1553 — CTAS CreateAsSelectSql (re-parsed under session mode by the internal executor)
• pkg/sql/plan/base_binder.go:2586
• pkg/sql/plan/apply_indices_ivfflat.go:368
• pkg/frontend/authenticate.go:10998,11182,11633

Suggested fix

Make WriteValue emit a MySQL-syntax single-quoted literal: quote-double ', escape \ per mode (consulting the FmtCtx.noBackslashEscape flag added in #25683), and never emit Go-only escapes. That makes all quoteString serialization mode-correct in one place, and lets FullTextMatchExpr's special-case formatting collapse into the common path.

Care needed: some callers may rely on the current %q output as an opaque key (e.g. semantic/cache keys) rather than re-parseable SQL — those should be audited before changing shared behavior, or the fix gated behind a new option consulted by re-parse-bound callers (CTAS first).

Discovered while reviewing #25683 (see discussion there).