Speedup pending txs list query

[vbaranov]

Motivation

The currently implemented query is slow because it doesn't use current indexes effectively. Current query execution plan:

explain select * from transactions where block_hash is null and error is null order by inserted_at desc, hash desc limit 50;
                                                       QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=7535181.93..7535182.05 rows=50 width=398)
   ->  Sort  (cost=7535181.93..7541132.96 rows=2380414 width=398)
         Sort Key: inserted_at DESC, hash DESC
         ->  Bitmap Heap Scan on transactions  (cost=107203.94..7456106.29 rows=2380414 width=398)
               Recheck Cond: ((block_hash IS NULL) AND (error IS NULL))
               ->  Bitmap Index Scan on transactions_block_hash_error_index  (cost=0.00..106608.84 rows=2380414 width=0)
                     Index Cond: ((block_hash IS NULL) AND (error IS NULL))
(7 rows)

Changelog

Change the order of columns in the sorting to utilize "transactions_hash_inserted_at_index" btree (hash, inserted_at) index

explain select * from transactions where block_hash is null and error is null order by hash desc, inserted_at desc limit 50;
                                                               QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.70..54264.67 rows=50 width=398)
   ->  Index Scan Backward using transactions_hash_inserted_at_index on transactions  (cost=0.70..2583414415.84 rows=2380414 width=398)
         Filter: ((block_hash IS NULL) AND (error IS NULL))
(3 rows)

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build 4cc1192d-046e-4174-872c-422c3a4b411b

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 75.095%

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Files with Coverage Reduction	New Missed Lines	%
apps/block_scout_web/lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	2	71.43%

Totals
Change from base Build e1bb55f3-1833-4488-81ba-238c7a276264:	-0.01%
Covered Lines:	5506
Relevant Lines:	7332

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💛 - Coveralls