cgosqlite: cache column name strings forever

[bradfitz]

The set of column names in a program is relatively bounded; cache them in memory forever to save on some per-query allocs.

Benchmark (of 5 row result set) doesn't show huge CPU/time win, but alloc reduction does show up. Questionable whether this matters, but I saw it in profiles.

name              old time/op    new time/op    delta
ReadStringRows-8    5.62µs ± 0%    5.67µs ± 0%   +0.88%  (p=0.000 n=9+9)

name              old alloc/op   new alloc/op   delta
ReadStringRows-8    1.62kB ± 0%    1.30kB ± 0%  -19.80%  (p=0.000 n=10+10)

name              old allocs/op  new allocs/op  delta
ReadStringRows-8      79.0 ± 0%      69.0 ± 0%  -12.66%  (p=0.000 n=10+10)

[crawshaw]

What benchmark are you using?

My memory is these are cached per-query at the level above in the driver. As long as you're passing in WithPersist contexts, these shouldn't be re-read on subsequent runs of the query.

[bradfitz]

This was wrong. It was supposed to be on ColumnName, not ColumnText. But ColumnName isn't hot (or even used) by us currently so not really worth fixing yet.