On Sat, Nov 12, 2016 at 11:49 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Oleg Bartunov <obartu...@gmail.com> writes: > > Hi there ! > > I don't know when exactly it was improved, but following notice in > > https://www.postgresql.org/docs/current/static/textsearch-controls.html# > TEXTSEARCH-HEADLINE > > is currently not needed. > > > ts_headline uses the original document, not a tsvector summary, so it can > > be slow and should be used with care. A typical mistake is to call > > ts_headline for every matching document when only ten documents are to be > > shown. SQL subqueries can help; here is an example: > > I don't see why that stopped being appropriate? The point is that it > takes a raw text input which has to be re-parsed; that's still true > AFAICS. > I mean that in the past we recommended to use subselect to avoid extra ts_headline() call, which now, at least at 9.6, it's obsoleted and two sql queries call ts_headline() exactly 5 times. select ts_headline(body,to_tsquery('supernovae & x-ray')),ts_rank(fts,to_tsquery('supernovae & x-ray')) as rank from apod where fts @@ to_tsquery('supernovae & x-ray') order by rank desc limit 5; explain (analyze, costs off) select ts_headline(body,to_tsquery('supernovae & x-ray')), rank from ( select body, ts_rank(fts,to_tsquery('supernovae & x-ray')) as rank from apod where fts @@ to_tsquery('supernovae & x-ray') order by rank desc limit 5 ) as foo; select ts_headline(body,to_tsquery('supernovae & x-ray')),ts_rank(fts,to_tsquery('supernovae & x-ray')) as rank from apod where fts @@ to_tsquery('supernovae & x-ray') order by rank desc limit 5; > regards, tom lane >
