Re: SELECT with LIKE clause makes full table scan

Hi,

On Wed, Jan 26, 2022 at 12:39:25PM +0100, Matthias Apitz wrote:
> 
> > > sisis=# explain (analyze, buffers) select * from d01buch where d01ort like 'Z 9610%' ;
> > > QUERY PLAN
> > > -------------------------------------------------------------------------------------------------------------------------------
> > > Gather (cost=1000.00..680262.71 rows=510 width=952) (actual time=1324.096..1349.429 rows=1 loops=1)
> > > Workers Planned: 4
> > > Workers Launched: 4
> > > Buffers: shared hit=102040 read=560674
> > > -> Parallel Seq Scan on d01buch (cost=0.00..679211.71 rows=128 width=952) (actual time=1117.663..1315.062 rows=0 loops=5)
> > > Filter: (d01ort ~~ 'Z 9610%'::text)
> > > Rows Removed by Filter: 1055853
> > > Buffers: shared hit=102040 read=560674
> > > Planning Time: 2.028 ms
> > > Execution Time: 1349.593 ms
> > > (10 Zeilen)
> > >
> > > Why is this (ignoring the Index) and what could be done?
> [...]
> create INDEX d01ort on d01buch(d01ort)  ;/*  D01ORT    */

That index can't be used with a LIKE that has a trailing wildcard.  You need to
either create an index with text_pattern_ops operator class (see
https://www.postgresql.org/docs/current/indexes-opclass.html), or a GIN index
using pg_trgm (which will also work with non-trailing wildcards), see
https://www.postgresql.org/docs/current/pgtrgm.html.