On Fri, 2014-07-11 at 11:51 -0400, Tom Lane wrote:
> Jeff Davis <pg...@j-davis.com> writes:
> > Attached is a small patch to $SUBJECT.
> > In master, only single-byte characters are allowed as an escape. Of
> > course, with the patch it must still be a single character, but it may
> > be multi-byte.
>
> I'm concerned about the performance cost of this patch. Have you done
> any measurements about what kind of overhead you are putting on the
> inner loop of similar_escape?
I didn't consider this very performance critical, because this is
looping through the pattern, which I wouldn't expect to be a long
string. On my machine using en_US.UTF-8, the difference is imperceptible
for a SIMILAR TO ... ESCAPE query.
I was able to see about a 2% increase in runtime when using the
similar_escape function directly. I made a 10M tuple table and did:
explain analyze
select
similar_escape('ΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣΣ','#')
from t;
which was the worst reasonable case I could think of. (It appears that
selecting from a table is faster than from generate_series. I'm curious
what you use when testing the performance of an individual function at
the SQL level.)
> At the very least, please don't call GetDatabaseEncoding() over again
> every single time through the inner loop. More generally, why do we
> need to use pg_encoding_verifymb? The input data is presumably validly
> encoded. ISTM the significantly cheaper pg_mblen() would be more
> appropriate.
Thank you. Using the non-verifying variants reduces the penalty in the
above test to 1%.
If needed, we could optimize further through code specialization, as
like_escape() does. Though I think like_escape() is specialized just
because MatchText() is specialized; and MatchText is specialized because
it operates on the actual data, not just the pattern. So I don't see a
reason to specialize similar_escape().
Regards,
Jeff Davis
*** a/src/backend/utils/adt/regexp.c
--- b/src/backend/utils/adt/regexp.c
***************
*** 688,698 **** similar_escape(PG_FUNCTION_ARGS)
elen = VARSIZE_ANY_EXHDR(esc_text);
if (elen == 0)
e = NULL; /* no escape character */
! else if (elen != 1)
! ereport(ERROR,
! (errcode(ERRCODE_INVALID_ESCAPE_SEQUENCE),
! errmsg("invalid escape string"),
! errhint("Escape string must be empty or one character.")));
}
/*----------
--- 688,703 ----
elen = VARSIZE_ANY_EXHDR(esc_text);
if (elen == 0)
e = NULL; /* no escape character */
! else
! {
! int escape_mblen = pg_mbstrlen_with_len(e, elen);
!
! if (escape_mblen > 1)
! ereport(ERROR,
! (errcode(ERRCODE_INVALID_ESCAPE_SEQUENCE),
! errmsg("invalid escape string"),
! errhint("Escape string must be empty or one character.")));
! }
}
/*----------
***************
*** 722,781 **** similar_escape(PG_FUNCTION_ARGS)
while (plen > 0)
{
! char pchar = *p;
! if (afterescape)
{
! if (pchar == '"' && !incharclass) /* for SUBSTRING patterns */
! *r++ = ((nquotes++ % 2) == 0) ? '(' : ')';
! else
{
*r++ = '\\';
*r++ = pchar;
}
! afterescape = false;
! }
! else if (e && pchar == *e)
! {
! /* SQL99 escape character; do not send to output */
! afterescape = true;
}
! else if (incharclass)
{
! if (pchar == '\\')
*r++ = '\\';
! *r++ = pchar;
! if (pchar == ']')
! incharclass = false;
! }
! else if (pchar == '[')
! {
! *r++ = pchar;
! incharclass = true;
! }
! else if (pchar == '%')
! {
! *r++ = '.';
! *r++ = '*';
! }
! else if (pchar == '_')
! *r++ = '.';
! else if (pchar == '(')
! {
! /* convert to non-capturing parenthesis */
! *r++ = '(';
! *r++ = '?';
! *r++ = ':';
! }
! else if (pchar == '\\' || pchar == '.' ||
! pchar == '^' || pchar == '$')
! {
! *r++ = '\\';
! *r++ = pchar;
}
- else
- *r++ = pchar;
- p++, plen--;
}
*r++ = ')';
--- 727,813 ----
while (plen > 0)
{
! char pchar = *p;
! int mblen = pg_mblen(p);
! if (mblen == 1)
{
! if (afterescape)
! {
! if (pchar == '"' && !incharclass) /* for SUBSTRING patterns */
! *r++ = ((nquotes++ % 2) == 0) ? '(' : ')';
! else
! {
! *r++ = '\\';
! *r++ = pchar;
! }
! afterescape = false;
! }
! else if (e && pchar == *e)
! {
! /* SQL99 escape character; do not send to output */
! afterescape = true;
! }
! else if (incharclass)
! {
! if (pchar == '\\')
! *r++ = '\\';
! *r++ = pchar;
! if (pchar == ']')
! incharclass = false;
! }
! else if (pchar == '[')
! {
! *r++ = pchar;
! incharclass = true;
! }
! else if (pchar == '%')
! {
! *r++ = '.';
! *r++ = '*';
! }
! else if (pchar == '_')
! *r++ = '.';
! else if (pchar == '(')
! {
! /* convert to non-capturing parenthesis */
! *r++ = '(';
! *r++ = '?';
! *r++ = ':';
! }
! else if (pchar == '\\' || pchar == '.' ||
! pchar == '^' || pchar == '$')
{
*r++ = '\\';
*r++ = pchar;
}
! else
! *r++ = pchar;
! p++, plen--;
}
! else
{
! if (afterescape)
! {
*r++ = '\\';
! memcpy(r, p, mblen);
! r += mblen;
! afterescape = false;
! }
! else if (e && elen == mblen && memcmp(e, p, mblen) == 0)
! {
! /* SQL99 escape character; do not send to output */
! afterescape = true;
! }
! else
! {
! memcpy(r, p, mblen);
! r += mblen;
! }
!
! p += mblen;
! plen -= mblen;
}
}
*r++ = ')';
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