Re: Request to help on Query improvement suggestion.

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Re: Request to help on Query improvement suggestion.

Laurenz Albe
On Fri, 2020-05-22 at 16:15 +0530, devchef2020 d wrote:
> PostgreSQL : 9.5.15

> Created Indexes on column parental_path:
> =================================
>
> CREATE INDEX cable_pair_parental_path_idx
>   ON SCHEMA.TABLE_NAME
>   USING btree
>   (md5(parental_path) COLLATE pg_catalog."default");
>  
> CREATE INDEX cable_pair_parental_path_idx_fulltext
>   ON SCHEMA.TABLE_NAME
>   USING gist
>   (parental_path COLLATE pg_catalog."default");

> SELECT seq_no + 1 FROM SCHEMA.TABLE_NAME WHERE (parental_path LIKE '%,sheath--' || cable_seq_id || ',%' OR parental_path LIKE 'sheath--' || cable_seq_id || ',%' OR parental_path LIKE '%,sheath--' ||
> cable_seq_id OR parental_path = 'sheath--' || cable_seq_id) ORDER BY seq_no DESC LIMIT 1;
>
> Explain Plan:
> =============
>
> Limit  (cost=108111.60..108111.61 rows=1 width=4) (actual time=4597.605..4597.605 rows=0 loops=1)
>  Output: ((seq_no + 1)), seq_no
>  Buffers: shared hit=2967 read=69606 dirtied=1
>  ->  Sort  (cost=108111.60..108113.09 rows=595 width=4) (actual time=4597.603..4597.603 rows=0 loops=1)
>        Output: ((seq_no + 1)), seq_no
>        Sort Key: TABLE_NAME.seq_no DESC
>        Sort Method: quicksort  Memory: 25kB
>        Buffers: shared hit=2967 read=69606 dirtied=1
>        ->  Seq Scan on SCHEMA.TABLE_NAME  (cost=0.00..108108.63 rows=595 width=4) (actual time=4597.595..4597.595 rows=0 loops=1)
>              Output: (seq_no + 1), seq_no
>              Filter: ((TABLE_NAME.parental_path ~~ '%,sheath--64690,%'::text) OR (TABLE_NAME.parental_path ~~ 'sheath--64690,%'::text) OR (TABLE_NAME.parental_path ~~ '%,sheath--64690'::text) OR
> (TABLE_NAME.parental_path = 'sheath--64690'::text))
>              Rows Removed by Filter: 1930188
>              Buffers: shared hit=2967 read=69606 dirtied=1

An index on an expression can only be used if the expression is exactly the same as on one
side of an operator in a WHERE condition.

So your only chance with that query is to hope for a bitmap OR with an index on "parental path".

Two things to try:

1)  CREATE INDEX ON table_name (parental_path text_pattern_ops);

2)  CREATE EXTENSION pg_trgm;
    CREATE INDEX ON table_name USING GIN (parental_path gin_trgm_ops);

Yours,
Laurenz Albe
--
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