I just rearranged the code in mbutils.c a little bit to make it more
robust if conversion of an over-length string is attempted, and noted
this comment:

 * When converting strings between different encodings, we assume that space
 * for converted result is 4-to-1 growth in the worst case. The rate for
 * currently supported encoding pairs are within 3 (SJIS JIS X0201 half width
 * kanna -> UTF8 is the worst case).  So "4" should be enough for the moment.
 * Note that this is not the same as the maximum character width in any
 * particular encoding.

It strikes me that this is overly pessimistic, since we do not support
5- or 6-byte UTF8 characters, and AFAICS there are no 1-byte characters
in any supported encoding that require 4 bytes in another.  Could we
reduce the multiplier to 3?  Or even 2?  This has a direct impact on the
longest COPY lines we can support, so I'd like it not to be larger than

                        regards, tom lane

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